IRO makes good stuff. This is a great bike to buy. You will get solid performance out of a reasonably priced bike.

As far as 631 vs. basic CrMo goes, there are some differences. 631 is a bit stronger meaning that it can be drawn thinner and is thus lighter. I have a bike made out of 631 (Jamis Sputnik), and I absolutely love the way it feels. Something about it seems a bit snappier, too. When i mash up hills, it seems like the bike kind of bounces off of each pedal stroke. I dono, maybe its all a placebo effect and I'm a product of clever marketing, but I can notice a difference between it and my standard CrMo 4130 ride.

Speaking of Sputnik, you might want to check one out. I like mine a lot. Salsa Casseroll is also nice, IMO. Steamrollers are always good (though I think they are overpriced). These are all more relaxed geo bikes. If you want something tracky, there's another list...