Judging by the character trailers, I'd say we get to see quite a fair bit of Havok and Banshee (much to my surprise.) And I'm sure there's much more, especially since there seems to seem to be a real family quality to the team. Obviously Beast and Mystique are the supporting roles, so we'll be seeing a ton of them. I'd say Angel, but she's in the CIA fight, the Cuba fight and (BIG SPOILER) [BLACKOUT]turns on the X-Men and sides with the Hellfire Clib in the final fight.[/BLACKOUT] She doesn't sound slighted to me. It's hard to judge Darwin as I've not seen much from him, but at this point, I'd guess him. But I was certain Havok and Banshee were going to be background characters until we got more trailers/footage, so, watch Darwin be a pretty significant role. Azazel and Riptide. All fight and little character, perhaps?
We can't deny Vaughn's pulling off an ensemble cast of mutants pretty damn well.