If I had my 'druthers, I'd play strictly with my thumb. Both acoustic and electric. When I sit quietly in my living room I can do it just fine, and my good old thumb has a great sense of rhythm and dexterity and, especially, control, and that's a great feeling. And so liberating not to have to worry about the pick. I can't exactly "shred", but I never could anyway; with my thumb only, I can navigate any rhythm and most lead lines (for the material I usually do) without any problem. BUT, when I go to play in any kind of setting where we generally need to project a little better, with or without an amp or mic-up, and it's time to use a little more aggressive attack, it just sounds so darn MUDDY! Especially thumb-strummed chords! I find this to be true, for instance, with the duo I have with a friend of mine. Personal practice is typically thumb-only, and I love it; but when it's time to play together, that just doesn't seem to cut it, and I'm fishing for the pick! I often wonder, if I just keep at it, and keep at it, that maybe eventually that sweet spot on my thumb will develop into a rougher surface and start to sound more crisp, and less mushy? Any thumb-only players out there who might have any encouraging thoughts?