http://www.theology.edu/biblesurvey/eccles.htm
This outlines most of the pros and cons to solomonic authorship i've seen discussed intelligently. personally, i shy away from ascribing it to solomon. one thing that's key to understand is that authorship was viewed very differently. to not ascribe works to one's mentor or inspiration was considered dishonest, while we would consider pseudonymity of this sort fraudulent. IMO, it doesn't really matter who wrote it...it's interesting and provoking.