While it doesn't offer much more than "more of the same", that's not really a bad thing.
The Quake III engine is getting slightly dated and this game doesn't offer much more than what we saw in Allied Assault and Spearhead, but that doesn't mean shooting up nazis and blowing up ruined buildings isn't a whole lot of fun.
Breakthrough could just as well be called a "mission pack", since the story here is just "go kill the enemy", as it was with the first two games. You've got eleven extra levels to travel through various locations (which are actually pretty neat -- the first level, for instance, you fight in a raging sand storm, barely able to see farther than the barrel of your gun), more bad guys to kill, different tanks and turrets to use, and just about anything you would expect from a Medal of Honor game.
There is also a new multiplayer mode called "Liberation", which sounds pretty neat, though I've not tried it myself.
Well, it simply doesn't add anything new to Medal of Honor. I, personally, don't consider that too bad of a thing, but that's really just my opinion. This does, however, mean that the flaws found in the first two games can be found here as well. Such as the way that whenever you man a gun turret, a horde of nazis will come charging haphazardly at you for the specific purpose of you gunning them down. Your squadmates are also freakishly bad shots, even at close range, and I don't think there was a single part where they took down an enemy by themself.
The Bottom Line
A mission pack, that's what this is. Just like those old Doom and Duke Nukem 3D mission packs that used to be all the rage, Breakthrough doesn't bring much more new to Medal of Honor except new levels and new locations. Whether or not this is a bad thing is purely subjective.
Personally, I enjoyed it.