I went and saw Dragonball Evolution this weekend, and I was quite entertained. Sure, it's pretty ridiculous and the characters are very 2D, but isn't that DBZ anyway? They did a clever job of getting a fair amount of characters and mythos stuffed into an 80-ish minute movie from what was originally a 42 volume manga! You can't go into the theater expecting Citizen Kane. My biggest complaint was that Goku never trains in 100x gravity! Maybe they're saving that for the sequel (which is very much hinted at at the end)....
Also, I feel I have to defend Super Mario Bros. here as it is my favorite anime/video game movie (with Speed Racer a close second). They explain everything one takes for granted in the traditional side-scroller: why he's called Bowser AND King Koopa, why it's the Mushroom Kingdom, Big Bertha being a man-eater, warping, the list goes on. And Mario's plumbing skills actually come in handy!
(In fact, I'm hard pressed to pick an anime/video game movie I hated; even Dead or Alive was fun, maybe Mortal Kombat 2 is the one to throw under the bus.)