I've never dived on a Mares reg, but I understand they're ok. Personally, I feel that Apeks and ScubaPro are the manufacturers to be most trusted.
In the end, your reg decision depends heavily on the conditions of its use (recreational, overhead, deep, etc.), its reliability and servicability, its comfort and convenience, etc.
It might be more helpful to us to tell us about your intended uses, your budget, and what has led you to consider the Mares M12 Proton. We can then help you determine if your reasoning is sound.
Some bad reasons to choose a reg:
1) It looks pretty
2) It has a swivel and eighteen LP ports!
3) It's made out of spage-ace technopolymercompositefibre!
4) It has a ruby!
5) It's #1 in such-and-such a catalog!
Some good reasons to choose a reg:
1) It demonstrably breathes easy, on a machine and on a human being, on the surface and at depth.
2) It is reliable, can be disassembled and reassembled with a minimum of tools, and uses easily available parts.
3) It does not vibrate or breath annoyingly
4) It does not blow its bubbles right in your face
5) It does not breathe wet (i.e. it gives you nice dry breaths, rather than leaking small amounts of water in)
6) It has diver-adjustable parameters like cracking pressure (this can help stop freeflow while it's not in use, and still provide top-notch breathing performace)
... there are more, but hey, I'm tired.
- Warren