I've been perfectly happy with mine -- for less than $200 from LeisurePro and my being a new diver, it's been great. However, I had a somewhat similar experience to yours.
After maybe 10 dives and less than a year of ownership, I started getting the low battery warning. I over-thought things and guessed it was because I had the date set totally wrong or something, and then the low battery warning went away by itself, but then I had a dive where it refused to go into dive mode, and the low battery warning came back.
I had a trip coming up fast, and the local Mares service center was temporarily closed, so I figured I'd just try a new battery. Easy do-it-yourself replacement, and it's been fine since (10-15 more dives).
If you're still under warranty, definitely give Mares a shout and see what they can do for you. If not, try replacing the battery and see if everything works fine again.
BTW, given that the Puck is a pretty old computer, but LeisurePro was still selling them super cheap when I bought mine, I'm wondering if they were selling a pile of old stock whose batteries were already getting a bit tired. Anyone else with really short life on the initial batteries of their brand new Puck?