Slevin has two pretty good things going for it; a plot with a few twists and turns, and a great cast. I think that the filmakers were going for a Tarantino-esque feel with the quirky characters, black humor, and oddball dialogue, and while the movie doesn't live up to those lofty heights, it does have a bit of style that gives the overall film a bit of a boost.
I initially only wanted to see this because it had Lucy Liu in it, and she was by far my favorite person in the cast. Damn, that woman is just beautiful, and she proves here that she can tackle almost any kind of role. Having Bruce Willis, Ben Kingsley, and Morgan Freeman in the movie as well makes for quite the nice bonus. They don't have to stretch or work particularly hard, of course, but just their presence here makes the movie more enjoyable.
Even Josh Hartnett handles his character quite nicely. He's the focus of 90% of the scenes, so if he was annoying or poorly cast, then the entire film might have been unwatchable. Luckily, that's not the case.
Don't watch this expecting a shining moment in film history, but it's certainly worth picking up for cheap or from Netflix. Lucky # Slevin is a twisty movie that has the ability to make you chuckle a few times before it's all over.