I've done the spontaneous worship thing before, but my current church arrangement (3 services every Sunday) does not allow for it due to time constraints.
And I'll echo what Mm said; sometimes it can be a wonderful worship experience, but far too many times as a listener I've discerned some self-indulgence on the part of the performers and witnessed the "us 4 and no more" experience where many in the congregation just check out and are done way before the band is.
That said, I dig a lot of what I hear coming out of Bethel and IHOP and Jesus Culture, etc...even Israel Houghton has those times of improv worship where you can easily get overwhelmed by God's presence.