I love my Mosin Nagants. I bought my first, an M38, about 6 months ago, and I just bought a 91/30 last weekend. I also have a CMP Garand, and a Yugo SKS, but I still find I shoot my Mosins a lot more than the other Milsurps. Part of that is ammo price and availability. Surplus 30.06 is still available cheap from CMP, but in a year, I've only gone through half a case. Its a great rifle, but I only shoot about 40rds each time I go to the range. 7.62X39 is getting pricey, which is why I don't shoot the SKS as much. 7.62X54R is really cheap, but the cheap stuff is corrosive, as others have mentioned, so you have to do a bit of cleaning at the range. Otherwise, the MN's wouldn't ever get cleaned, because they don't need it.
I can't speak much on what to look for on the SKS or the Garand, but on the Mosin you really want to check out the bore before you buy. Its common to find pitting in the bore because the Russians didn't care about shooting corrosive ammo and not cleaning the rifle. Just open the bolt, pull the trigger and slide the bolt right out the back, and then aim it at a good light source and you'll be able to see all the rifling, and pitting if there is any. Also find out what comes with the rifle. Many will come with a sling, some ammo pouches, a messy oiler can, and 91/30's will come with a big spike bayonet.