I just used an M-S pair yesterday to reamp bass tracks for an instrumental post-rock project I'm doing. The band is a three piece: drums, piano and bass. There is a lot of space for each of the instruments to take up and the bass is often playing lead parts. I set up two Traynor 8x10 bass cabs on either side of one 4x10. I then connected whatever guitar and bass amps were on hand in the studio to those cabs...a Bandmaster, a Dual Showman, and a Carvin bass amp. The reamped DI signal was fed into all three amps simultaneously. A U195 and an R121 in M-S about 3 feet away nicely captured the size and power I was going for.
I've also had good luck with M-S for drum room mics.
Brad