Killzone has a free MP weekend this next weekend
, and you can pre-load it now. Maybe check it out first?
I'd say the MP is worth it for $35 alone; in additional all competitive MP DLC released will be added for free as well (except for any "COOP DLC").
Personally I liked the story and thought reviews were overly harsh in most of their complaints, especially stacked up against other AA/AAA FPS's (They're docking KZ for it's SP while giving BF and COD pretty weak campaigns a slide). It's NOT a GOTY game, but it's got a very serviceable, but forgettable, SP. MP is very solid for those looking for a different sort of MP shooter than COD or BF offer (much more focus on accuracy in the shooting, and teamplay for the scoring).
Should be a KZ thread around here to page through for some deeper thoughts.