spitter is the noobs choice i feel, as it's easy to rack up points as long as you wait for something else to attack.
hunter, you need to know the high places/skybox for a nice 25 damage, but also really useful at taking out a wandering buffoon.
boomer, vulnerable, but it's just about being sneaky.
smoker, pretty useless unless you get a death smoke point, or everyone else is in trouble/occupied with horde/another special infected, or if you get someone close to incap as you can do a ton of damage.
charger used to be awesome before the hitbox seemed to get bugged completely. it's only really useful now if you sneak up or do the quick-turn style charge.
tank, well if you're up against a decent team and you haven't got backup from your team, or if you're just bad, it's rubbish. if you're sneaky, good with rocks, and have good players on your team its a decider for the round.
jockey... well, a lot of people diss the jockey as it's pretty feeble e.g. gets 1-hit killed if you're in the air. i think it's not bad, it's handy when most/all the survivors are boomed, as you can get someone into a lot of bother far away from the other survivors. it's still my least favourite as sometimes it just won't connect with a survivor, no matter how accurate you are. again, this could be a similar thing to the charger with the hitbox being fubar'd