What's worth grabbing in the Sonic and X-Men sales on CMX? Already have what I want from the Rebirth one.
I can answer on the Sonic stuff. Pretty much the rule of thumb is to just pick up stuff from Ian Flynn. He jumps on the main series at #160 and stays on ever since. It's worth noting, though, that I feel he certainly still does improve over time, and at least at the start he's largely cleaning up the crap that was there from past issues. So that certainly may not be for everyone, to say the least. If you want an easy jumping on point, there's a reboot from 252 onwards (coming out of the first Sonic/Mega Man crossover). Oh, and Universe is also solid stuff, largely focused on self-contained arcs (based on sets of 4 issues to fit nicely into trades... which they've stopped making), and often focused on characters other than Sonic. I REALLY wouldn't recommend touching any of the stuff pre-Flynn unless you have a REALLY strong nostalgia for it or something, as it's easily some of the most poorly done and stupid comics I've ever read. AKA god damn is Ken Penders awful.
So yeah, for most people I'd probably just recommend jumping on at the reboot starting at issue 252. And of course this stuff probably isn't for everyone, but if you're willing to read a Sonic comic, then Ian Flynn certainly knows what he's doing, and you should be able to have some fun with it. (In case it helps further, Ian Flynn is also the guy who did the sadly hiatus-ed Mega Man comic).
Edit: I guess it's important to note that it IS still Sonic, though, so it's best if you're coming in with an interest/acceptance of what that brings with it in the first place. IE I don't think it's on the level of More Than Meets the Eye, where I'd recommend it even to people who have an aversion to the franchise. It's well done, but it's certainly still Sonic. So if you're not particularly interested in the franchise, I'd recommend treading lightly, and maybe give the first arc of Universe or the reboot a try before diving in too heavily.