I just want to warn you that there are going to be a lot of people chiming in with various experiences on the Remington 870 Express. They range from "once I cried while in the same room as my 870 Express and it rusted through and crumbled into dust" to "I was sailing around the world and lost my anchor so I used my 870 Express as an anchor AND paddle while firing 50,000 rounds and not a single jam or spot of rust." It's going to be really hard to listen to the qualitative feedback you get, unfortunately. Just try to take every piece of advice with a grain of salt.
I myself have had 2 870 Expresses and have had no problems yet... but I have not yet put thousands of rounds through like others have. I also work at a range that has a couple of Expresses and one crapped out recently (the other is going on strong) but it was a few years old and keep in mind it was a range gun... at a range that doesn't require a PAL so it wasn't treated like an owner would (hopefully) treat theirs. Oh it didn't completely die, I don't know what the problem was but it was fixed so it didn't even crap out beyond repair.