Anyone who has a RADAR detector is going to tell you that it's worth it, because everyone wants to feel like they didn't make a bad purchase. They think that any time the RADAR detector beeps and they slow down, that was an avoided ticket. The reality is, you can drive past dozens of cops while speeding and not get any tickets. Cops don't pull over every speeder.
Some things to consider -
1. In my experience, there are things that make you more likely to get pulled over. Don't do anything to draw attention to yourself. Speeding doesn't draw attention to you - everyone speeds. I drive ~80-85 on the interstate and don't ever get pulled over. Driving significantly faster than surrounding traffic draws attention to you. Frequent and abrupt lane changes draw attention to you. Following closely draws attention to you. Having a car with flashy external modifications draws attention to you. Having out of state plates draws attention to you, because you're less likely to contest the ticket.
2. Cops don't have to keep their RADAR on. They can turn it on when they see you speeding, and by the time your RADAR detector warns you, they already have your speed. Maybe you get lucky and your detector picks up the RADAR when he's checking someone else's speed, maybe he doesn't. Just realize that your RADAR detector is useless when there is no RADAR to detect, and the absence of RADAR does not mean you're not going to get a ticket.
To me, a RADAR detector is a waste of money. If I had a riced out car and drove like a jackass, it'd probably be a worthwhile expense. But all I do is speed (within reason), and I don't get tickets for it.