A little too easy, and the co-commander gets annoying. Except for when it's Autumn Reeser.
I had fun playing as the Soviets, and Tim Curry never got too annoying for me, but then, once I started to play Allies, it all became a bit repetitious. It said somewhere - maybe in the tutorial (which was exceptionally painful) that you shouldn't use the same strategy over and over again otherwise the computer will catch on - I didn't find this at all. I found that you could use the same units over and over again, no problem. The game is a little too easy. I played it on Medium, but it kinda seemed like easy. And I'm not that good at RTS as a rule.
The other thing I didn't like was the co-commander. In some situations, the co-commander was fantastic - helpful, resourceful, etc. In others, annoying. I'm a slow player. I like to build up a large army, take the time to make my base well defended, etc... I think the problem I had was that I would be building up a force, while my co-commander was just flying around blowing **** up. And so by the time I was ready to attack, there wouldn't be much left. And yeah, you could tell me that I should just play faster, but I don't want to play faster. I like to take my time. But there didn't seem to be an option to like, make your co-commander just sit there and mine ore for you while you spend it on tanks.
Other than that, I've had fun with this game so far. The plot was a bit predictable, but good, the acting in the cut scenes was solid (especially from the aforementioned Autumn Reeser). Tim Curry and what's-his-name from Tomorrow Never Dies as well. So yeah, there are lots of fun things with RA3 - but I don't see it as a game I'm going to be playing over and over again.