Germans Love David Hasselhoff: Naturally, it did well in Asia (outside of Japan where it lost to Yatterman), going number one early on before dropping off. It should be noted that it was number one for several weeks at Asian box offices, and while the reception was mixed it is still rather popular and well-liked outside the US.
Ham and Cheese: James Marsters as Piccolo and Chow Yun-Fat as Roshi were awesome casting choices.
The movie itself earned a bad reputation with Dragon Ball fans and generally the anime and manga fandom as a terrible Hollywood live-action adaptation of a very popular anime franchise. Because of this, the fandom would remain skeptical on Hollywood's attempt on adapting manga and anime into the big screen. The reception of Edge of Tomorrow might have changed this view but perhaps not enough.
The writer, Ben Ramsey, finally admitted in an open letter to Dragon Ball fans that it is an Old Shame of his, so much so that what little of a film career he had was utterly destroyed by the film (as his IMDB page demonstrates).
Shocking Swerve: Sure, we understand that Goku is not of this Earth, but The Reveal that Goku is Ozaru comes directly out of nowhere. After Piccolo's reign ended, how can Goku still be alive and who sent him into space only to send him back to Earth? And even then, there is no explanation for why Piccolo went to such lengths to have him killed if he was so critical to his plans.
Snark Bait: Fans of Dragon Ball just love trashing this movie. Just mentioning the film in a Dragon Ball forum post is bound to start a flame war. Even the people who worked on the English dubs of the Dragon Ball anime love to make fun of this movie at fan conventions. Critic and Dragon Ball fan Chris Stuckmann has even called this his least favorite movie.
So Bad, It's Good: Most of the reactions were negative, even from those who never saw the animated series. That said, it can be enjoyable. A small minority think it was a decent film in its own right, to the point it could be considered an okay film had it not had the Dragon Ball name, and as stated above, a lot of people in Asia love the film.
See what Goku's head collided with when grandpa Gohan knocked him off? Sure, he was falling pretty fast, but melons shouldn't be THAT soft, should they? Also in the previous scene, one of Goku's kicks didn't even touch his grandpa. Force Kick in effect? The 2 consecutive failures can be seen in one video.
In Goku's case, in the series, he was known for having a rather hard head, to the point that he got Bulma reflexively shot him in the head during their first meeting and he only berated her for hurting him.
There's also that one scene with Goku and Piccolo in a Ki blast struggle… that ends with Goku shooting himself towards Piccolo with no propulsion whatsoever and in the same Kamehameha firing pose.
There's also a bad case of Styrofoam Rocks towards the end of the movie. You'll know it when you see it.
And let's not get started with either the obviously fake ape suit substituting for Oozaru or the wretched CG used throughout.
James Marsters didn't appear to take his role of Piccolo too seriously in practice; however, he did give several long-winded speeches on the character's motivations, referencing William Shakespeare in one of them and treating the role as a Composite Character of Piccolo and Kami (who were technically the same being anyway). Still, he gives a downright subtle and restrained performance compared to most everyone else in the film.
Marsters: He used to be a force of good, but he was imprisoned, making him very angry, and then he escapes... The cool thing is anybody who's seen Dragon Ball knows that Lord Piccolo transforms into THE Piccolo, and that is a whole other ball of wax; heroic wouldn't be the wrong term, but it's a long journey.
Perhaps a better fit for this trope is Justin Chatwin as Goku. Holding back other considerations of how his character's motivation are changed from Anime to Film, his performance of Goku as an insecure teen is pretty good, even adding some character development as he becomes self-confident to the point of gaining Heroic Willpower.
Vindicated by History: Downplayed. This movie was not a good first impression of actor Justin Chatwin. Due to a well received guest spot as a superhero on Doctor Who and his role on Shameless (US), some viewers are more forgiving of his role in this movie. Some is the correct answer.
What An Idiot: Why did they spend hours in a hole when Roshi had superhuman jumping powers?