Supposedly the employees that worked at 3D Realms (the original developer that had been working on this for so many years) managed to continue working on it last year after being laid off, and they actually had the PC port completed end of last year. And supposedly GearBox is just polishing the PC port and then creating XBox 360 and PS3 versions, so there's a bit of inner grumbling about GearBox taking more credit than deserved.
From the comments I've read it sounds like it's fun to play, but graphics a tad dated-looking. That's no big surprise considering how long it's taken, but looks aren't everything. I'm sure with this much development time and attention to detail it's going to be good.