...you'll probably get mauled to death - probably not a nursery rhyme that will really catch on, but it does tend to describe the film. Piranha 3D (note: I didn't watch it in 3D) is about - you guessed it - a horde of nasty flesh-guzzling fishies eating hordes of scantily-clad teenagers. In fact, fat people were obviously not allowed on set during this production.
There are bad films and there are good bad films. I'm pleased to say this is the latter. You should basically know what you're getting - no Shakespearian dialogue or massive character developments or plot twists, just daft, silly (and very gory) fun.
However, it's worth mentioning that just because this film was very `popcorn watchable,' not every film in the same league is. I've also watched the sequel `Piranha DD,' and, in short, it's rubbish. Whereas this one was a lot of silly fun, the second time around it feels forced and there's nothing new.
If you're looking for some grisly fish action and bikinis aplenty, then you have found your holy grail.