i bought this game for $40 when it first came out and thought my money was well spent. i've beaten the game on all difficulty levels and have all of the guns and stuff. a lot of people have a different opinion of it tho.
It may be short, but i played that thing to death, beat it like 20 times on various difficulties and its still fun! ive been wanting to play it again, but there are just too many new games coming out. for cheap, sure go for it IMO, i thought it had some great SP replayability.