Is Suros Regime worth spending my Strange Donuts on?

I'd say it has more to do with your play style & usual weapon preferences than anything else. SUROS is a great auto rifle, but if you don't typically use ARs, then I doubt this one will redefine your play style. I've had two of the old 300 models for months, and neither of them have ever been leveled all the way, just because I don't use them enough. I'll always prefer scout rifles. Autos make me feel like I'm wasting ammo every time, and the drop in range between most autos and even a crappy scout is truly significant. SUROS is a pretty nice middle ground between the two, particularly once you buy the early upgrade that slows its fire rate when aimed down the sights, but I've found that it's nothing you can't improve on with a sexy scout rifle in your hands. That said, if you don't have Red Death (which is considerably better since the pulse rifle buff/auto rifle nerf), then the health regen perk will pay for itself in Crota HM runs. And SUROS sure does sound great when you fire it - like one of those pneumatic nail guns. Plus, there'll always be more strange coins, but Xur doesn't sell that one very often, so if there's any chance you'll beat yourself up later over letting it get away, then I'd go ahead and grab it anyway. You never know when Bungie will rebalance and bump auto damage back up again, either...