We did IoA and USF a lot last year on our WDW trips. Had annual passes there and also at Sea World. This year we got AP's at Disney. Don't know yet how regular we'll swap which years to have AP's at which parks.
US and IoA are nice, but they don't have quite the same 'magic' as Disney. I think US has slipped a lot over the years. They just haven't kept it really 'fresh'. For example, while Disney's Epcot has their great ToN parade and illuminations, Universal has discontinued their water show at park closing time.
I think a lot depends on personal preferences and how much time you have each trip.
On 1/31/01 our AP's for US/IoA, Sea World, and Disney all overlapped, so, just for the fun of it, we did a power day....all 7 parks in one day. (USF, IoA, SW, MK, MGM, AK, Epcot). If anybody's interested, I did a page on our home page. (the address is in my profile)
Caskbill