I can't say how much HP you get from it, but I can definitely tell you it kicks a**. I just installed one this weekend and I love it. Before the VAK, my torquless wonder duratec would get a chirp, maybe two when stomping it from a dead stop. I did a test run yesterday in 80* weather(Florida winters are brutal, aren't they?) and got a nice 1-2 second solid tire screech. The VAK is definitely worth it, and I would recommend to anybody. Also, it's something you can install yourself, unlike the UDP I got from ShoShop.
Also, on gen 3's, it cuts out the alternator, power steering, and a/c(I think).
It is neither time consuming nor irreversible. It's like intalling a head unit. It doesn't take too long, but you need to cut some factory wiring to do it. You do need to tap into a vacuum line for the "trigger" switch, but in most cases, that vacuum line is overdue to be replaced anyway, so it's no loss.
Everything is controlled electronically. The PS load is reduced via the VAPS electronic circuit. As mikehawk said, we can't really spell it out for you, b/c that would be unfair to Paul Nimz, who developed the VAK.