Minor spoilers ahead:
Bryan Singer kicked off the X-Men movie franchise incredibly well before running off to do more damage to Superman than Doomsday ever did. While the first X-Men movie wasn't perfect, its strengths far outshine its flaws. The casting was near-flawless, especially when it came to Xavier, Magneto, and Wolverine. They were even stronger in X2, Singer's followup. X3 was a total train wreck, killing off major beloved characters with all the emotional impact of a hiccup. But at least Wolverine was still cool.
X-Men Origins somehow manages to take everything that worked about that character in the first two-and-a-half movies, and tosses it out the window. How they managed to take an already well-established character and make him more two dimensional while giving him more screen time - I'll never know. He's mostly humorless, and when he does crack a joke it's not very good. Maybe he doesn't gain a sense of humor until he loses his memory. That might also explain the recurrence of his flipping-the-bird-with-his-claws routine. You can repeat the same joke if you don't remember doing it the first time (or if you're a lazy screenwriter).
The movie's one redeeming factor is that it's full of unintentional humor. I'm not enough of an X-Men fan to be blinded with rage by such a wrong-headed adaptation, so I managed to enjoy every moronic twist that came along. When a girl shows up at the end of the movie who has "skin as hard as a diamond", nothing prepares you for the visual representation of this when it finally shows up. Imagine someone who's been bedazzled from head to toe and you get the picture. Lots of the movie is filled with things that the filmmakers thought were going to be awesome, but are in fact incredibly stupid. The worst is the conclusion of Wolverine's walking-into-the-sunset moment. You know what's about to happen, and it's still dumber than you imagined. If you go into the movie ready to roll with this, then there is entertainment to be gleaned from it.
One of the weakest parts of the movie is the special effects. This movie did not have a slashed budget, but it looks like it was made for television. Maybe this is because the director has mostly worked in television, I don't know. But after three movies that (at the very least) looked really good, Wolverine's claws look fake for the first time. They decided to forgo physical props for CGI and it is glaringly obvious. I swear, it's like the filmmakers weren't even aware that there were previous X-Men movies. But the absolute worst thing is the appearance of Professor Xavier at the end. What is supposed to be a "Holy Shit Awesome!" moment, is instead a "Holy Shit, Monster!" moment. Because what we see is a version of Patrick Stewart digitally altered to look like a young man, done in such a way as to give the impression that a wax sculpture has come to life and is hungry for souls. At least that's the impression that I got.
Other things that suck:
Every other character in the movie. After waiting years for Gambit to make an appearance, X-Men fans are getting a really shitty version of him. Think Angel in X3 - just no real point to him being around at all. No one is developed beyond their function in moving the story.
The dialog is laughably terrible throughout.
Wolverine's childhood is included at the begging, why? It has absolutely nothing to do with the rest of the movie. I imagine they included it so the movie would at least have some small resemblance to his origin in the comics. It introduces a mystery involving his father that is never mentioned again, than jumps straight into the opening credits sequence which is as dull as anything else in the movie.
Ultimately I feel Wolverine is another step downward, even from X3. But I actually enjoyed watching it more, since I expected it to be lousy.