The Z1 is an excellent reg. My wife and I both use them and I never notice it underwater, it just works. The best thing about the Z1 is that it performs as well as the other Atomic regs and costs substantially less. The higher-end regs have some bells-and-whistles that the Z1 lacks, but you can add the bells from the higher-end regs (the M1 exhaust T and the B2 swivel) to the Z1 for less than the increase in cost to get the higher-end regs (because you're not paying for the higher-cost materials in the B/M/Ti series). We have the M1 T, and it's great. They didn't have the B2 swivel when we got ours, but I've been thinking about adding it. We'll see.
Amusingly, if you look at the weight charts, the Z1 is actually the second-lightest reg in their lineup, after the Ti's, which are 100% titanium.
Service costs should be pretty low if you have the unsealed model. If you want to do your own service, they are covered in the Airspeed Press books, although I don't know how easy it is to get spare-parts kits for them.