Man, I'm ambiguous about this one. I rarely play campaigns in RTS games, I dive right into the skirmish mode. I'm not sure I'm a huge fan of the "levelling up" system, although the "card" system is okay, I don't really think it was necessary to make it so that you have to play to obtain them. You could just do any strategy you like from the beginning.
I guess it was alright. I played it a lot at the beginning, mostly playing coop against AIs with my girlfriend. It was fun, I love the way buildings fall to pieces, the feel of battle and the sound effects were great. The sheer number of units fighting was fun too. But in the end I felt that the game maps felt a little contrived or too small and homogenous. All the maps pretty much felt the same. When I was playing AoE2, I somewhat felt that my army was crossing fast lands or something, in AoE3, it's feels like the enemy is basically on the other side of that patch of wood there, and there's nothing blocking them. I don't know how to explain it, there was something about the maps and their size and homogeneity...
Also, it's a much more offense oriented game. I find it's a trend in more modern RTS games, and I'm not sure I like it. Even Sins of a Solar Empire was largely based on offense. And just zerging your enemy out. Many of these games don't leave me much time to contemplate (although MAYBE Sins of a Solar Empire does... Unless you're called out at three different places..)