I have a couple of cheaper units (a Cobra, and an old Bel), and a Passport 8500xi. They all do a reasonable job of informing that I am about to get a ticket about a second before the flash goes off, or approximately two seconds before I can hit the brakes.
I have paid out more money to purchase radar detectors than I have paid out for tickets. At least twice I have paid for tickets while having an expensive radar detector in the car that didn't warn me in time.
The best radar detector in the world is still my eyes, and with the optional accessory of my brain which is not always reliable, but it works better than any device that I have ever paid money for.