I just bought a Whistler XTR-140 off of Amazon.com. It's clearly an entry level Radar Detector ($40), so I'm not looking for too many bells and whistles, I just want it to warn me if I'm being scanned. Have any of you had good or bad experiences with this unit? I've passed at least two cop cars so far, and the Whistler hasn't chirped a bit. I get stray X from driving around town, but no K or Ka. Do cops usually keep their radar guns off? Should I trust this unit to warn me?