I'm a very happy Atomic owner (I have 5 sets of M1s, a singles set of Z2/M1, a Z1 argon reg and a few other Atomics I need to sell), I'd recommend them for the following reasons:
They are the only regs I know of with a 2 year service interval - if you rebuild your regs as the mfg. recommends, the Atomics will cost less in the long run, not to mention you will face the inconvenience of bringing them in for rebuilds only half as often. (Time is worth something too, right?) I believe the 2 year interval is largely because of the seat-saver orifice feature.
They are a simple proven design with a excellent reputation and track record. Their quality and attention to detail is excellent. I find them a pleasure to use; in the regulator shoot-offs I've seen, they consistently ranked #1 on a breathing machine.
With a LP port on the bottom of the turrent, they make for very clean and versatile hose routing.
Because, for the most part, all the regs in their line are based on the same design, they are very upgradable down the road. You could get an entry level Z2 swivel and completely convert it to an M1 with a few parts, if you get into tech diving down the road. Or you can just pick and choose a few goodies like the excellent M1 exhaust deflector, heat sink, environmental seal kit, cave ring, second stage swivel, etc...
They are pretty simple regs, easy to work on, rebuild, and adjust yourself, if you choose to go down that road. Finding parts kits will be the hardest part.
They are made in the USA, and have a lifetime warranty - NOT dependent on getting them rebuilt at regular intervals, unlike some companies.
#1 reason for choosing Atomics - the comfort mouthpieces rule!
Rob