Not a masterpiece by any means, but it's a fun shoot 'em up that never outstays it's welcome.
The dialogue is relatively funny and engaging, and not for the faint if you like your films politically correct. Especially the lines coming from Willem Dafoe. The performances are decent enough, even if the two leads(Reedus and Flannery)can't really stick to their Irish accents.
The Boondock Saints does exactly what it says on the tin. If you're expecting Quentin Tarantino tier dialogue or masterful direction then look elsewhere. You watch this film with a few beers and some friends, it was made for that.
You can tell Troy Duffy had lots of potential, but it never really came to fruition because of his abrasive and arrogant behaviour at the time(see the documentary "Overnight").
As it is, it's deserving of it's cult status and you should definitely check it out at least once. The sequel? Whatever.