I like human personally, for the following reasons....
1) They have a great mix of spellcasters/aa/melee/air
2) Great defenses (ie: Watch towers, cannons, and that new one .vs spellcasters)
3) The co-op building makes for quick buildings under pressure
4) GREAT combination of heros
Etc...
But there are some major drawbacks....
-Footmen are weak, so you can be beat early on in the game easy
-Like Swooper said, requires a lot of microing if your going casters
-Slow resourse gatherers (slower than undead or NE at least)
-Requires MANY food suply buildings (farms are only 6, whereas all the other races are 10, i think....)
And a few more...