A lot of good points here. Most people come from the improve path it seems. Improv is good, but I heard Steve Vai once say that he learns his solos so amazingly well through practice and practice and practice, so that when it comes time to play, he barely has to think about it and can just let the music take him away. Sometimes when pros look like their just playing from their gut, they're really just playing what it took them hours of practice to learn. Just a thought...

I think good solos should contain a good mix of memorized licks, and that the structure should be planned so as to be most effective against the background, but hell this is music, so you don't need to learn it note for note. A classical player would disagree however.

Of course improve is always fun, but that's more of a "jamming" thing (which is good too) then an actual solo over a song, imop.