I have one too. Once in a while I clean the probe with vinegar and recalibrate. It doesn't drift much though. I think they're really very accurate.
I do have two big complaints with it:
1) It's not waterproof. Once it stopped working because it got a little saltwater in it. I took it all apart, cleaned it, let it dry completely, and reassembled. It works again now, but what a pain.
2) It doesn't shut off automatically, so I'm frequently running the battery down.
I really wish that hanna made one of those little waterproof pens for high range salinity.