Not a bad game
Since this is Blizzard's first 3D game, the graphics are not at all bad, but large battles can be slow sometimes. The heroes are definately a good addition to this game, since they can become quite powerful if you know how to play with them. The units are really balanced and unlike most other strategy games (such as C&C), the units of different races not only differ in colour, and names, but also looks and abilities, making each race have truly unique units. Not only are the units different, each race also has different abilities. For example, most of the Night Elves' buildings can move around and they can see better in the dark, while the undead can only build on blight (similar to the zerg creep in Starcraft). Excellent singleplayer gameplay and has the most incredible cut-scenes I have ever seen, even better than Diablo II's cut-scenes. As with most other Blizzard games, clicking on a single unit for a couple of times makes them say something funny, and since there are so many different units, you can listen to a lot of these comments. WC3 also features a great map editing application. With it, you can do just about everything, from creating your own heroes to the ability to detect key strokes from within the game. Also, now 12 players can be in the same battle for multiplayer, so you can have even more intense battles.
This game is just too similar to Starcraft and probably not as good. Mobile buildings, the presence of blight, and how the Undead Ghouls look suspiciously like Zerg Zerglings all looks too much like Starcraft. In fact there are even Starcraft units hidden in the game! Another thing I dislike about this game is how strong the defense towers are. There are many people on the internet who doesn't do anything else but build towers, hundreds of them. Since the food limit is 90 now (so that your strategy now becomes a key to winning), you can only send a few units in to destroy the towers, but most units die without even reaching the towers while catapults could easily be taken down by aerial units within the mass of towers.
The Bottom Line
If you have not played Starcraft yet, buy that instead of Warcraft 3. However, if you are a fan of Blizzard's games or just strategy games in general, then you should buy this. If you figure out how to (and have the time to actually do it) make maps using the map editor, you could make complete different games with it (puzzle, RPG, etc).