Better than any expansion I've seen
What makes a good expansion? If your answer is a couple new units and a few multiplayer maps then your expectations were severely lowered by most modern games. If your answer is just as many multiplayer maps as the original, even bigger and better campaigns than the original had to offer, new extremely powerful units, heroes whose special abilities dwarf any other and a whole new town, then you just described HoMM 3: AB.
Since HoMM 2: PoL
, New World Computing
has established a high plank of expectations for their expansions. This expansion for HoMM3 continues that proud tradition of doubling the original content by adding many new singleplayer and multiplayer maps. Sadly those maps aren't centered around other new additions (like the new powerful dragons and heroes, nor even the new town), and are played similarly to the old ones. You can choose to play as the new Conflux town, but other than that you only have the new designs to keep you interested.
A truly great addition was the new Conflux town. While, arguably, the weakest town and one that is the hardest to develop (as it has rather costly units and building upgrades) it offers the best magic heroes in the game. Starting with higher knowledge/power than Warlocks or Wizards, Elementalists are the only heroes to start with an elemental magic skill. And if that wasn't enough the special building of the Conflux is the Magic University which teaches all four of the elemental skills. The end result is a hero that easily dominates the battlefield with their magic expertise.
The town creatures are actually quite reliant on their hero's magic, as they are often weaker than their counterparts in other towns. In their unupgraded state they are all weak melee fighter that need mass statistic boosts (like master level Bless, Iron Skin, Haste, Bloodlust, etc.) to hold their ground. But once upgraded two of those weak melee units become strong (relative to their level, of course) shooters that change the battle mechanics dramatically, turning the Conflux into a force to be reckoned with.
Since the town employes elemetals their forces have an unique characteristic of being well protected against some spells, but very susceptible to others. This means that this is one of the few times when your heroes will want to use the protection from element spells (luckily, the upgraded versions of your troops are also casters that can cast protection from their element on any unit).
The best feature of the expansion is naturally the campaign mode. It offers more maps than the original, the maps are harder and make full use of all the new features. The main expansion campaign, that is also its namesake, is by far longer and more interesting than any other HoMM3 campaign. In it you get to play as elves, knights, their demon enemies and finally as the mysterious Conflux. In that setting you get to know why the new town appeared, employ new units like the very powerful sharpshooters (units that can both purchased from out-of-town structures and trained using one hero's special ability) and the not-so-new halflings.
Other campaigns are much more specialized as they prefer to explore one side of one story of one faction within the game world. These stories are unconnected to the main plot, but very enjoyable as they touch faction that were less prominent in the original. Warlocks, barbarians, witches (the swamp-dweller's Fortress campaign is the only one, beside AB to touch the main plot of HoMM3) and wizards all get their spot in the sunlight with four medium or large sized maps.
These stories follow the exploits of a single hero with very specific special ability. The wizard, just like the elven hero from the main arc, can train original units into a new, more powerful unit. The witch has the special ability to start with fire magic mastery (perhaps the most useless specialty, but it does make narrative sense) while other main heroes specialize in their 7th tier monster, bolstering their power considerably.
These campaigns are more difficult than the original ones and offer greater challenge to the player. The wizard campaign also makes the most out of the new dragon types and pits the player against overwhelming armies of dragon as the player tries to complete the mission on time. The fact that some heroes can train their own units adds new strategical dimension to the game, which is a very welcomed diversion from the usual.
The expansion also offers two new artifacts. One of them empowers dragons and is useless for most towns, but the other one is the dreaded Armageddon's Blade. An artifact of such awesome power that it is used only in campaigns.
The game now also allows you to recruit both the upgraded and the unupgraded creature from an upgraded dwelling (useful when you're low on gold or resources). But most importantly you can now leave defenders on your mines, thus completely negating the usefulness of weak mine-stealing heroes. This new feature is very useful in multiplayer games, forcing your opponent to readjust their resource gathering strategy.
Additional features include the random map generator - a rather unpopular feature as its maps are utterly unbalanced and some towns have inherent advantage there. The other feature, an actually useful but rarely used one, is the campaign editor. This program allows you to "tie" your edited maps into your own campaign the same way the regular campaigns work.
While there are few fan made campaigns out there, it is nonetheless a very welcomed feature.
This expansion offers everything an expansion can offer without being a new game or a spin-off. It's really hard to ask for anything more, and so the only problems come from the original (reviews for the vanilla version describe them well enough) and are not inherent to the expansion.
There are, however, some small issues. All new heroes are overpowered compered to the standard heroes and therefore unusable outside the campaign. Particularly the two heroes that can upgrade their own units. This means that those heroes are disabled for standard maps (something that can be reversed with the editor that allows you to include or exclude any heroes or artifacts), which defeats the very idea of having new heroes in the first place.
Similarly, new units aren't used nearly as much as they should be in the standard maps. These maps are so poor in new content that they could have been released for the original without anyone noticing the difference. The new dragon dwellings are so insanely overpriced that even when they are included on the map you have no reason to recruit them at all. Again, this defeats the purpose of having powerful creature dwellings - instead of turning the map into a game of king of the hill for the creature dwelling, the players mostly ignore it and carries on with the main objective. There simply is no reason to make a Rust Dragon cost about four times more than a Black Dragon when its statistics are only about 2.5 times better.
The last criticism is regarding the Conflux. It's nice to have a new town, but you must get it balanced just right to be usable. Unfortunately the unupgraded units are pathetic when compered to other unupgraded units. Upgrading those units is mandatory if you want to defeat your enemies, but there is a reason why most players never upgrade most structures - it's both costly and time consuming. The turn you use to upgrade is a turn that could have been used to build a new structure, not to mention that the gold would be better used to buy creatures. Additionally, the town relies heavily on mercury as its main resource. If you start in map that has poor access to mercury, or on a high difficulty setting that decreases your starting resources, you could run into troubles much faster than towns that require mostly wood and ore. Conflux does have the strongest 2nd and 3rd tier units in the game, but they are also much more costly than similar units and they hardly compensate for the rest of the army, which is way below average.
In a way the Conflux is very similar to the Necropolis in that it has costly shooters and weak melee units, as well as being immune to Blind spells. However, without the necromancy skill to bolster their ranks, and with high susceptibility to certain elemental spells, playing becomes a challenge.
Sometimes the only way to effectively play with Conflux is by using fire immune units (it has three of those) and blasting the battlefield with the Armageddon spell. However such strategy is very situational as you have a small chance of finding an Armageddon spell, but will spend huge amounts of resources searching for it.
The Bottom Line
A must have expansion. While the new additions may not be much on the multiplayer side, the campaign more than make up for it. Simply put - if you enjoyed playing the original campaign, the new one will be pure ecstasy. And even if you didn't you still might want to give Armageddon's Blade a try as it is harder, more engaging and interesting than Restoration of Erathia.