I use it, and I love it. You'd have to be crazy to pay the $200 new retail price for a new instant film back, you can get one on ebay for less than half that cost. I got mine in a used camera shop here in Japan for about $40.
I don't use an instant back for instant gratification, but for insurance. If you are shooting 4x5, then you know how much work it takes to set up and compose a shot. A shot on instant film lets me know if my exposure settings are correct, rather than having to wait until after I develop my film.
Fuji's basic instant films are very cheap and easy to use, and they give great results.