+:
it is a lot quicker to remove the filter than to clean the lens
they are easier to clean (always flat, almost no dusty barrel around, cheaper to replace after you scratched it by cleaning it the wrong way, which btw is more common than you might think)
protection in the field (insurance does not pay for the pictures you lost)
-:
expensive (i use b+w)
iq:
it does always degrade the iq on certain lenses (very rare), it sometimes degrades iq on any lens (almost never). i have a couple of very good lenses and tested it in a lot of situations, there was no difference at all*, but it can never hurt to try for yourself.
*on good monitors @ 100%. technically, there might be one, but if you cant see any, who cares.