#1
I've been playing for a fair while now, about seven years, probably four or five seriously. I consider myself a decent player - and in my band I improvise pretty well. But I just realized that in the last year or so, I've only learnt about one or two set in stone short solos.

Does learning established solos offer any benifits in terms of development or is working on improv phrases and such what I should be doing anyway?
#2
I think that ultimately learning to improv well is a lot more important.
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#3
You could do both...just figure out what key the solo you learn is in and improv during the solo. Spice it up a little, give it your own touch.
#4
Man, not for me. I stopped learning "solos" a year ago, once I became fluent in my improvisation. Screw it, I got no time to waste on it unless I absolutely love the solo. I do however learn shreddy stuff, all classical, bach, paganini, ect, both for repertoire and technique. So I say, if you got your imrpovisation down, just make your own solos.