How bad is it? Interesting premise, poor execution.
Should you see it? Not really.
In this film, a volcano doesn't erupt to kill people, it reveals itself as a monster; a monster literally the size of a mountain, and perhaps all of the earth, may be a record for film. Unfortunately, it's never seen well - the CGI isn't good and the film is murky - and being hidden doesn't help, as it sometimes does in better films (Val Lewton's for example). This SyFy film is further hampered by wooden acting and some plot holes. For a simple, monster rampage movie, it doesn't match 1950's films.