I have a pair of IIIa bodies---both with accurate meters, incredibly---that have seen quite a bit of "regular camera" use. I haven't really had a problem with the ergonomics, though clearly some people do---OK, knob rewind is a pain in the butt, but otherwise, no issues. The fixed viewfinder means that with anything other than a 50mm lens, you effectively have a separate rangefinder and viewfinder, which is livable but intrinsically slower than a combined window. I mostly use 50mm anyway, so it hasn't been an enormous issue for me, but someone who favored other focal lengths would have more trouble.
The meter adds bulk, but I suppose less than carrying a separate meter would. There aren't going to be a lot of working, accurate meters out there, though; unless you can confirm meter accuracy before buying, it probably makes more sense to go with the meterless IIa.
The big thing is, OH MY GOD THE LENSES, especially the 50/1.5. There are technically better-performing lenses for 35mm, but to my taste that lens has the perfect balance of technical virtues and "character", especially in color. The 85/2 is also a real winner. I don't have any of the "native" wides, just an oddball third-party 28mm, but people say wonderful things about the 35/2.8 Biogon (however, the prewar version and the Soviet/fSU copies are reported NOT to fit on the postwar bodies).
I'm moving away from 35mm a bit lately, but the Contaxen are on the short list of cameras I'm pretty confident I'll never unload. I'm actually rather tempted by the Voigtlaender R2C; it doesn't have the giant rangefinder base, but it has TTL metering and framelines and would provide a modern, less eccentric platform for the lenses.