Well, I'll first say that I'm new to diving, with only about 8 months of experience. but I tried a ScubaPro MK25 first stage with a S600 2nd stage and a G250V, and both are awesome, and breath quite well. (I've only tested so far, but will let you know more in a little bit) I haven't had any experience with other systems, but I love this reg! I have noticed that many brands (including ScubaPro) often ding you if you buy a more advanced 2nd stage as a backup, without buying a 1st stage,(the other option is buying a not as nice octo reg, without certain adjustment capabilities) but I think that a little more is definitely worth it. I'll say it again, I don't have a lot of experience, so my comments will definitely not be the best out there!
P.S. I know this is common knowledge to most of you, but I thought I'd mention it also---a piston 1st stage is a definite choice for cold water diving...