1) No idea how common they are - some folks have them, but the majority? I doubt it.
2) It can be worth it if for no other reason than for the discounts you get from Brownells, Midway, etc. The more stuff you buy from them, the better a value the C&R is.
3) It's an easy process, yes.
4) You need to keep all your C&R firearm transactions logged in a bound book - both purchases and sales, with the name and address of the person you bought from or sold to for each firearm. This bound book has to be kept for as long as you maintain your C&R license, and can be subject to audit by the BATF. This was the one thing I didn't care for when I had a C&R, having to maintain the bound book. I'm not that organized