What made the original Spinal Tap so great was that it was both comedy improv gold, and also way too close to home. Many bands, expecially R.E.M, The Misfits and Nirvana said that their touring experiences were just Spinal Tap in real life. Both Ozzy Osbourne and Eddie Van Halen found the movie too depressing rather than funny. On its pre-screening, a lot of people said the movie was good, but Reiner should have picked a more well known band. Spinal Tap feels like an out of touch rock band, with egos to boot, and that's where the ridicculousness comes from, rather than artifically putting them in stupid and it seems more relevant scenarios.
The problem with a sequel is that we're in on the joke now, and it seems from the trailer that's the angle they're playing. And I'm also extremely wary of real life cameos. Real bands were mentioned in the original, but the movie largely exists within its own universe. They have played live and had shorts with real bands before, but I don't think I could stomach an hour and a half of it. As a fan who rates the original a literal 11/10, and has watched the 4.5 hour wortcut print, I will be waiting a long time and for a very good review before watching.