I've never ridden one, but have seen a few and the people I talked to who owned them said the drivetrain loss is nowhere near as bad as people claim. One of them was a TOC bike, so I'm not sure if the comparison is to TOC bikes or modern ones or what, but one was a newish Dynamic bike, which I just looked up and is based in the UK. Other than the interesting shaft drive, it looked like a pretty basic goofy newer hybrid with cheap components. Again, I can't speak for their experiences to know what they were comparing them to, but they liked them. I would really like to try one myself.
If you just wanted parts, that Dynamic bike and probably some other cheap one I never heard of might be a good start to cut out the parts and weld into a better frame. I love the weird things, even if they're not as good (like steam cars and radial engines), because it's not always about what's best, it's about what's fun and different can definitely be fun (when it's not a nightmare a . . . and even sometimes it can be both).