I have a Geo (as a back-up to my VT3) that was manufactured 06/2007, I bought it 10/2008 w/the original battery. It has 56 dive hours recorded on it and it still seems to be going strong. I do however carry a back-up battery for it in my save-a-dive.
In terms of Oceanic's reliability, I did have a VT3 flood on me. However, I think that had more to do with the shop I bought the computer from. I believe their policy is to install fresh batteries before they ship the computer to the customer (great policy) and someone did that, but forgot to re-install the O-ring (oops!). It's replacement has the same 56 dive hours on it that the Geo has and also has had no issues. It has given me "Deco-tention" on a couple of dives, but I haven't experienced it changing the altitude setting afterward.
There have been issues with batteries though. I can't recall the specifics, there are threads about it on Scubaboard, but some people experienced short battery life when they installed batteries that were the same type that Oceanic reccomends, but were not from Oceanic. They were bought at the local camera shop, pharmacy, Radio Shack, etc. AFAIK, people that have bought Oceanic OEM battery replacement kits have had no issues with battery life.