I do it much the same as others have mentioned. I usually start with a BIAB framework, brought into Logic or Digital Performer as a MIDI file, then layer audio tracks over it. I used to always replace the MIDI, but these days, sampled instruments are getting good enough that I can sometimes mix them with real instruments, after a little tweaking.
The "one man band" versus "real" musicians thing is something I still have mixed feelings about. If I were putting out a product for sale, I wouldn't do it the way I do now, I'd find good players and pay them to be on my project. But as it is, I do it for fun, and the amusement of a few friends, relatives, and whoever on the SGF that might dig it. I kind of like the challenge of doing all the parts myself.
Here are some of my recent jazz projects:
The Common Touch
Seven Come Eleven
Both feature some rudimentary C6 swing, and six-string guitar solos. On "The Common Touch", I wanted a piano solo with Bebop lines and block chords, but I don't play keyboard, so I had to struggle for hours playing the parts on a MIDI guitar synth to come up with anything that vaguely resembled jazz piano. A good pianist could have done something better in one take, but it was fun making it, and now I'm thinking of getting a better software piano.
So, after reading through this thread, I'm wondering, what do you guys do with these recordings? Aside from the professionals marketing a product, are most of us just doing it for fun, or to make them available for download, or just to drive your spouse crazy? I'd like to hear some of the work that others are doing. Charles Tilly's site has some good amateur recordings, and I have one up there. Anyone else want to post a link to some of your recordings? There must be some good stuff out there that not many people have heard.