Reuters reviewed Piranha 3D (I’ve not seen it yet) so read their review here.

“Jaws” it ain’t — Aja exhibits little patience for such stuff as dramatic tension and tautly coiled suspense, and there are some undeniable choppy bits — but he never loses sight of the potential fun factor laid out in Pete Goldfinger and Josh Stolberg’s script.Nor does he stint on the gore, with terrific assist by the veteran effects duo of Greg Nicotero and Howard Berger, who truly outdo themselves with all those nibbled body parts, as well as added bite by creature designer Neville Page.
With all the key elements in place, the 3D conversion, while decent, really doesn’t bring all that much to the party, especially one that also is attended by Ving Rhames, Adam Scott, pinup Kelly Brook and an irresistibly over-the-top Christopher Lloyd, pulling out all Doc Brown stops as a wall-eyed marine-life expert.