How bad is it? If you aren't paying attention, you could miss how bad this is.
Should you see it? Sure.
I've actually seen this film five times in a week, along with the other films in the series, and it's possible to enjoy it on its small merits as part of the franchise. That is, until you start laughing at David Boreanaz' hair... and you will; it changes a few times, but he sometimes looks very goofy. When he's trying to be sexy, a hipster Satan, you notice that he's getting a little old and soft for these roles. Then add Tara Reid, Ed Furlong, Emmanuelle Chriqui, Danny Trejo, Macy Gray and even Dennis Hopper, and waste all their talent. The new setting in the Southwest adds nothing except for some embarrassing racist elements (non- Native Americans doing fake tribal dances around totem poles 1000 miles from where they should be). Sometimes the cinematography is good, but there's really no plot or characters and it doesn't go anywhere. And then you notice Boreanaz' hair again.