It's your typical Musou game, for better or worse. I played the first two, not the third.
The first one kinda sucked gameplay-wise but had a very fun crossover story mode. Second one was much better but lost Japanese voices and the crossover thing was limited to very basic interaction in missions.
Third one was supposedly nice, but has only 00 and Unicorn as units for their series, with the rest being DLC. Dunno about the gameplay or crossover aspects.
That last part's not entirely true. In addition to the 00 Riser and Unicorn, they also put in the Reborns Gundam and Sinanju - it's a protagonist/antagonist setup for both shows, just like most of the other shows. It's true that they did add other suits to the game as DLC, though.
The main plot is a crossover, like the first game. Unfortunately, most of the dialogue is done with talking heads instead of cutscenes, but the few cutscenes that are included are
passable. There are a bunch of History, themed, and 'what-if" missions as well, so I'd say that overall Gundam Musou 3 strikes a balance between the first two games when it comes to story.
And I'll add a response to your post as well, _dementia.
One of the things that I like about the Gundam Musou games is that they're a bit faster than the other games in the franchise, thanks to the fact that the suits can just boost across the fields. Gundam Musou 3, specifically, is probably one of the fastest paced Musou games, since the maps are smaller and they added catapults in certain sectors, which makes traveling to different ends of the map easier.
Also, you can cancel attacks with dashes, so you can start a chain and then tack on a dash at any point, even at the end of it (I think, not entirely sure). It's useful for reducing recovery time, moving away from enemy swarms or starting up another chain, and as long as you don't overheat you'll usually be able to keep moving.
There's other stuff to the series, but off the top of my head I can't think of anything else.
And on a personal note, I feel better about the move to Gaming Community. I wanted to post in the thread more often, but I felt cagey about when it would be moved; I didn't want to be the guy who got the thread moved, if that makes any sense.