If you are shopping for an 870, try to locate one of the older Wingmasters. They were finished better, inside and out. You should have little problems from them.

The newer ones (and especially the Express guns) have some QC issues. First thing I would do with a new one is replace the plastic mag follower with a metal one (cures a lot of the feeding problems).

All in all, the 870 is still probably the most reliable current production pump shotgun, and the great thing is that there are a ton of aftermarket products for them too.