I played coop with a friend last year. The main problem I ran into with the multiplayer is that it seemed to want to use the public internet IP address to connect to each other, even though we were on a LAN together. We ended up having to unplug our internet router so that the game would stop detecting the public IP and connect using the private IPs.
We also had to disable all network controllers except the one we were using to play (meaning I had to disable my VPN and wired adapter so that only the wireless was enabled).
Kind of a pain, and we completely abandoned our attempts to play it over a VPN over the internet, but worth it. The above comments about the multiplayer being unstable are true (our experience wasn't that bad, but we typically crashed 2-3 times a session) as are the concern about the atmosphere losing something in the translation (you're not alone). In addition, the lack of respawn points towards the end of the game got frustrating and boring as each death resulted in a long hike through the same area over and over. Still well worth it, and we're talking about wanting to play it again (maybe after Borderlands 2).
This. Disconnect from any devices providing DHCP addressing and manually set IP addresses on your internal network it becomes possible to play in a LAN setting. I have finished the game a few times with at least one buddy by using this. As others have said, save often, save in different slots (quick save before bulkheads) as the game WILL crash frequently. The problems it has seems to increase with the number of people in the game. I have not been able to get past Deck 3 with more than 2 people, but two people can play all the way to the end if everything is done right.
Don't forget to disable firewalls after setting manual addresses. I wouldn't even bother trying to play this over the internet. It's just going to be a massive pain in the rear. But is it worth it? Oh hell yes.