I first heard about this film when somebody said it was one of the worst films ever made, so naturally I had to see it for myself. And I have to say I was disappointed, but not in the way you would expect. It's not so bad that it's unbearable, but nor is it so bad that it's good (like say Ed Wood's 'Plan 9 From Outer Space').
Yes, there is a lot wrong. The pace is completely messed up. The action continues on quite quickly, whatever is happening, which makes a change as most films with pace problems are far too slow, but ti means that emotional high and low points aren't given the attention they need to make an impact, there is no suspense and no build up making the ending very disappointing. It's also obvious that the actor's didn't have any fight training, and some of the acting is awful - none worse than Ben Kingsley though who manages with incredible skill, to bring absolute zero depth to his character and reels off his lines like he's reading a laundry list. Then there are other problems like the wholly unecessary flashbacks (especially the later one) and the info dumps put in so the audience aren't wondering whatever happened to the plot.
All that said, it could have been a lot worse. Loken fits the bill perfectly, capturing Rayne's elegance, beauty and ferocity very well. They even got her look right. The action is entertaining enough (if only because it's stupidly gory at times), but because of the fast pace it washes over you quite quickly and it ends up being moderately enjoyable despite it all.
In summary, its worth watching if you're a Bloodrayne or action/vamp. fan but don't expect too much.