Just got a Passport 8500 about a month back, and it paid for itself the very first weekend. Avoided 3 speed traps in as many days - if I hadn't had it, I would've been bagged for sure.
Doesn't mean they're fool proof - I've still been caught with a detector. There is very little defense against "instant on" radar or laser. And once, on a very long straight road in Maine, late at night, a cop got me at nearly a mile away. I just didn't think they had that range - learned the hard way.
But I'll say I've avoided a ticket about 80% of the time with a detector, some common sense, and knowing "where they hide". I just can't imagine driving without one. They should come standard with BMW's.