First of all, I want to say that I watched this movie knowing exactly what I was going to see, quality-wise. It doesn't try to be an earth-shattering masterpiece and that's why it doesn't fail as much as it could have. It is obviously influenced by Jaws, only features a different type of fish (and more of them -oh, and less suspense), and draws its strength from B movies from the '80s, so expect a lot of vibes from that direction.
This review includes moderate spoilers, but you won't be taken by surprise if you're familiar with this type of movie.
Premise
The premise is simple: an underwater earthquake unleashes a school of dangerous prehistoric piranha. Coincidentally, this earthquake happens during spring break, when the lake it took place in is full of horny teenagers. By now, you can probably tell where this is going: chaos ensues and said teens become piranha food, often in the most gruesome ways.
She has no idea what she's getting her pretty behind into. Literally.
The Good
There aren't many good points I can talk about, but one of them is definitely humor. And that's precisely the strong point of the film: it doesn't take itself seriously. We all know the standard horror movie formula: teenagers are trapped somewhere, killed off one by one and only a few lucky ones survive. The stereotypical characters are present as well: we have the Average Guy, the Spunky-But-Average Girl, the Tough Mom, the Hot Babe and a few others.
The gore factor is pretty high: teens getting eaten alive by piranha, which is shown on-screen. Just a heads up if you plan to watch this: Aja doesn't hold back on blood and guts here. These scenes are very graphic, ranging from teenagers with only torsos left of them to cops with half-eaten legs and bones showing.
Did I mention boobies? When you have a "director" filming Wild Wild Girls (a nod to Girls Gone Wild) and several busty women around, you know there's going to be a lot of nudity involved and the movie certainly delivers it. There's even a scene featuring two of the "actresses" swimming underwater, featuring full female frontal nudity (alliteration ahoy!). I can't even say it was tastefully covered nudity (contradiction again?), because the director has no intention of hiding anything. Including a penis, in one of the latter scenes. But read more on that in The Plain WTF, below.
The Bad
Unfortunately, one of the film's good points is also its downfall: it takes itself way too unseriously (I don't think this is even a word, but meh). Making fun of something is OK for a while, but then it gets boring. The shoddy backstory and major events contribute to this as well, as they are extremely generic: natural disaster happens, monsters are unleashed, people are stupid enough to let themselves get eaten, all-knowing Science Expert Guy uses his two minutes to explain the danger and is successfully ignored etcetera.
The director intended for the ending to be a twist, but I don't know if I should even call it that since it's been used so many times it has become ordinary. Its structure is: characters manage to escape danger, Science Expert Guy calls and tells them a shocking truth he discovered, therefore rendering everyone screwed to Sunday and back. Oh, and a Lovable Character dies as a consequence of this new-found truth. (I don't know about you, but I actually cheered when this happened because I was too hyped up and oxygen-deficient from so much laughing - even if I was kinda rooting for the poor bastard).
Also, some scenes are plain overkill, such as a ship's explosion and the massacre that obviously ensues when the dumb teenagers refuse to listen to the nice police men and get the Hell out of the water. And, no matter how amazing the human body is, I doubt one can survive for more than a few minutes after having his entire lower half bones picked clean of everything on them (and still manage to think of a wet T-shirts contest, on top of that). But, since they contribute to the overall comedic part, I guess you can take them either way, good or bad.
The Plain WTF (heavy spoilers concerning one less-important scene)
This part may gross out the more squeamish of you, so I suggest you skip it if you are offended by sexual themes or have an extremely vivid imagination.
There's one scene that definitely takes the cake for The Most WTF Moment Of The Year. Derrick (the porn movies director) gets his lower half nibbled by the fishies, and upon being saved remarks that "they ate his penis" (or something very similar to that, I was laughing too hard to hear correctly). A few moments later we see the aforementioned penis floating in crystal clear water and immediately eaten by a stray piranha, which proceeds to regurgitate half of it after realizing it don't taste well at all.
Verdict and recommendations
I watched this with a friend and we had fun, I'll admit that. That's why I highly recommend you watch it with someone so you can poke fun at it together, because that's what it was made for. Also, we watched it at home and I'm glad to have done it that way, since I for one couldn't keep my mouth shut throughout this thing (and neither could my friend). Oh, and you'll probably be in hysterics at some parts, depending on how twisted your sense of humor is.
Again, watch this if you want some light fun and don't feel like thinking too much. Shut your brain off and you'll enjoy it. Think too much and you'll be bored.
My rating: 
(4 1/2 out of 10)
Piranha 3D (2010) on IMDb.com
