Well, not too long ago, RA3 Uprising was released on digital download! Woo-hoo! I managed to try it out and play a couple of missions. (OK, that's a lie, I finished the relatively short campaign.) I believe most of the meat in the game is in the Commander's Challenge, which I'm not going to play until later.
The campaign is simple and short - 13 missions total. Then again, the Tale of Yuriko doesn't really feel like missions, so there are 10 new campaign missions to play through. I'd say a lot of them are standard RA3 fare - start off, fight your way through to gain a foothold, whack everything else in sight. I can't say much about how good they are, but they're a good way to get a look at the new (and most certainly IMBA) units like the Mortar Cycle, Pacifier and Giga-Fortress.
The Tale of Yuriko is interesting, putting the game into a dungeon crawler-esque mode where you have 4 abilities to use against your enemies. The story is ultimately pointless, but the campaign is fun nevertheless. What else could be better than whacking everything with a single unit? It even comes with boss battles! (Well, sort of.)
I haven't played the Commander's Challenge, where I believe most of Uprising's content is actually at. I guess that'll have to wait until after my exams. :D