I just watched The Room (for the first time) and The Disaster Artist back to back. It was like watching a movie and then all of the DVD/Blu-ray extras (that were more interesting than the film, itself). Compared to The Room, The Disaster Artist felt like a masterpiece and I enjoyed it, but I wouldn't say that there's anything remarkable about it (though Franco's performance is pretty good). It's your standard biopic... except with a more off-kilt subject.
The closest comparison is, obviously, Ed Wood, but that had more mass appeal and didn't require you to have seen Plan 9 from Outer Space. This is pretty much just for "fans" of The Room, IMO, since there are so many scenes (especially re-enactments of original scenes) that would probably just seem weird, out of context or disturbing if you're not already familiar with them. If you know all of the scenes, though, it can be pretty interesting to learn what was going on between takes and why scenes ended up as they did.
I wouldn't recommend it to anyone who hasn't seen The Room, but, for those that have and can find entertainment in that awful film, The Disaster Artist is a perfect companion film.