It can give you some advance warning, but it's nothing compared to just being alert.

I agree that being an alert driver is the single best way to avoid tickets. Spot them before they spot you.

And if you're going to speed, I would argue that you MUST be an alert driver or you don't have any business speeding at all. Several years ago I read a book that went into great detail on the subject of speeding tickets, and this was its central thesis.

Having a radar detector is simply another additional tool that can save you from a ticket in certain situations that being alert cannot. I got my radar detector after being nailed in a situation where I was being alert, yet the cop was in a hidden "trap" location. If I'd had the detector at that time, I wouldn't have gotten the ticket, yet no amount of additional alertness on my part would have saved me from the ticket.

It's also possible for them to nail you other ways other than radar, and some of them are much more ironclad, in court, than radar. The book I read went into excruciating detail on these methods. One of the advantages of owning a detector, they said, was that you would be able to know with more certainty what method they were using to clock you, and it can help you make an informed decision on what to do at any given moment. Even (especially) after they've already pulled you over. If you know how they nailed you, you can make a better decision about how you might want to try to talk yourself out of the ticket. It can be done, and it helps to have all the facts ahead of time.