I used their front set; no problems, just don't use the wheel studs they send (too long). You will need to get a proportioning valve, 15 inch wheels with more backspacing (about 1/2 inch) and some different brake flex lines. You should upgrade to a dual master cylinder.
As far as the installation goes it's on the easier side. (IMHO)
I have them at all 4 corners and I'm happy with them. I am getting rid of the pads that came with mine (they call them "coldstoppers") and going with some Porterfield R4S pads. I drove a friend's car at Thunderhill: he had the SSBC discs on the front, drums on the rear and, with the Porterfield pads, his car stopped very well.
As 1966conv said, you will have some parts that you end up buying in addition to what you get in the kit.
When I initially installed them, I did have a problem with excessive runout (i.e., the rotors weren't as flat as they needed to be). Wilwood sent me a new set of rotors for the front and I had the rears shaved just enough to smooth them out.
They are also probably more work than something like the SSBC kit. For instance, you need to safety wire the rotors to the hubs and proper torquing of all bolts is critical with all that aluminum. Also, Wilwood says they are strictly for racing: not for the street (sounds like the lawyers made them say it!).
I would put them somewhere between the SSBC street setup and the Baer in terms of performance. I don't have unlimited funds so I went with Wilwood. Baer brakes look great and I'm sure they perform very well, but, in my case, they would have been overkill (and they are very expensive). Even when I really gave them a workout on the track, the Wilwoods have performed well for me.