I have the Atom 3.0 w/transmitter. I have 40 dives on it and like it a LOT.
I have not had any trouble with losing the tank pressure display during a dive.
The Atom 3.0 and 3.1 are identical except that the 3.1 has the Buddy Check feature removed due to a patent infringement lawsuit. If you search around here, you can find a post from an exec at Oceanic that posted that info.
I've had my Atom since December and, just last week, found that Oceanic makes a "dome cover" for the computer that covers the entire display, including the bezel. And the dome seems to magnify the display a bit under water, which makes it even easier to read. The computer comes with just a flat screen protector that only covers the actual display itself, but does not cover the bezel at all. I got one and used it last weekend diving off the Outer Banks. I like my computer even more now.
As someone else already said, the alarm is not very loud. I hear it sometimes, but there are other times where I think it sounded and I didn't notice. AFAIK, I've only had it give me alarms to warn me when my depth exceeds 1.2ATA ppO2 when I'm diving Nitrox. It's set for a "limit" of 1.4 ATA and as a result I think it beeps a warning when you exceed 1.2.
My local dive shop was clearancing the Atom 3.0 for $550, with the transmitter and data cable, when I bought mine. If you can find a deal like that, I don't see why you'd hold out to spend twice as much for a SeaBear H3 with transmitter. I am looking to buy an H3 when the AI transmitter comes out, but that's because I have started Tech training. If I were sticking to only Rec diving, I would be perfectly happy to continue with the Atom as my main computer.
I looked at the OCi and the OC1 and I just couldn't see (ME) paying the extra for either of those over what the Atom gives you. An electronic compass? Thanks, I'm taking a real compass anyway. Titanium? Don't care. I'm going to put a protective cover over it, whatever it is, which means it's not going to look very swanky for just casual wear.
I thought I would be very careful and never scratch my computer on anything. But, a couple of tank swaps while on board a dive boat in choppy water showed me that, sooner or later, my computer was going to get scratched by a tank next to the one I'm mounting my BC on.