Seems the only people who'd speak favorably of this movie are either fans of the band featured or people who grew up with it. For perspective, I saw this movie when it first came out, admittedly, I'm no tween girl, but dedicated media just limits your appeal.
The movie feels like it exists solely to promote a band who were due to release their first album. Though something tells me that's exactly so. It was made in conjunction with their manager and their record label and the band's first album came out a few months after this movie got released. Effort was concentrated solely on getting the name out there, and it shows through a story that's thinner than a stick figure.
The girls play fictionalized versions of themselves, which isn't a bad idea in itself, but they go for over-the-top personalities that encompass their entire character. The key to making any good character is multiple dimensions to their personality. They do try this, mostly, for one character, but it's one that's somehow both too obvious and too undeserving.
Any character that isn't the main one with some significance is either too bland or too obnoxious to like.
Also at times I feel as though the movie was rushed. Action sequences and even establishing shots at times when they aren't needed are replaced with cheap animation. As if their filming permit expired or they just wanted this movie out as soon as possible.
Compared to other musical films, this sucks harder than others because of how corporate it feels. They cared more about pushing a brand than they did making something worthwhile. Longshot had a plot of some kind, The Naked Bros. Band had charm, Big Time Rush's means of comedy helped suspend your sense of disbelief.
Basically saying tweens would enjoy this movie suggests you have no faith in their sense of quality.