I have a CHP buddy who set up a mock "speed trap" and we tested a Bel STI, Rocky Mountain Radar, and a V1... the STI gave earlier warning than the V1 consistently at our test area by approximately 1/2 mile in both rear and front detection. The Rocky Mountain was a POS.
The V1 arrows are really, really nice though, and they're constantly doing firmware updates.
Also, for laser (which is becoming more and more common) any detector will only let you know you probably just received a ticket. By the time it goes off you have already had your speed read. You'll need a jammer for that (which are illegal in California.)
If you have a laser jammer and your detector goes off, it is advised that you slow to the limit and then switch your jammer off. If the officer never gets a reading off your car he'll probably become suspicious and pull you over.
My other experiences have been cops are far less likely to "cut you slack" if you get pulled over and they see you have a detector-- because "you don't need one unless you're doing something wrong."