How bad is it? It has no plot and the lip-synching of songs is off.
Should you see it? If you want. If you really really want.
Brainless fluff that's aimed square at their fan base, this is a plotless exercise to capitalize on the popularity of the Spice Girls. It follows the model of Lester's films of the Beatles and has quite a cast: Elvis Costello, Alan Cumming, Stephen Fry, Bob Geldon, Bob Hoskins, Elton John, Hugh Laurie, Meat Loaf, Roger Moore and George Wendt. If loud, frenetic, meaningless girl power isn't your thing, you'll hate it. Otherwise, it's not terrible.
It does go on too long. It is interesting, however, in retrospect, to see how Posh and Scary, the then least-popular members, show minor star power, which lasts to this day.