How do they compare with film?
I have yet to see a digital file that is as film-like as the P21+... especially at somewhat higher ISO. I mostly use a P45. But when I shoot the P21+ again and again I am very impressed by its very nice look.
Are those 22mp "fat pixel" backs that people used to buy (at those days) at prices similar to a good quality family car still worth buying?
Currently I have a P45 and P21+ (on Contax 645 and Cambo WRS), a Sony A7R, a Panasonic GH2 and GH3 (well, actually just sold the GH3) and some pocket cameras. If I had to choose only one single system today I'd most likely choose a tech cam with a 33MP or 39MP back. (Or, of course, a more recent back... but that was not the initial question...).