I went the first year they had it and it was a lot of fun. Good oysters, good music. But, the last time I went, there were so many people crammed into downtown Wellfleet that it was not fun. People were shoulder-to-shoulder. I couldn't wait to get out of there and Wellfleet is one of my favorite places on the planet. If you like crowds, I say go. If you don't, go to Wellfleet when it's not so crowded and you can take it easy. As a matter of fact, I'm there right now and the weather is perfect.
I've been going faithfully for the past 7-8 years ( and live in Boston). This year's weather looks promising, but keep an eye on it. It is always much more crowded on Saturday than Sunday. This is the Wellfleet O-fest and they serve Wellfleet oysters exclusively (which is fine by me), just don't go expecting variety.
Lots of different food options including beer and wine. Be sure to check out their website ( http://www.wellfleetoysterfest.org/) They have done a pretty good job of ironing out the logistics over the years. It's best to park in one of the remote lots and take the shuttle/ school bus ( free).
As John mentions, there quit a few high quality craft vendors as well as folks hawking stuff like hot sauces/ jams/ dressings. Clearly I'm a bit biased, but i think it is worth the trip.
Steve, have been a few times - though not in ~ 3 years now. The festival is fun, truly about oysters - all kinds, raw cooked grilled etc.; form different farmers bays etc. Its a small festival so feels more like a small town carnival (without the rides) than a big crowded event, which we liked. There were some nice artists and craftspeople (not the junky kind) there; in fact my parents bought a print to frame. Overall, I'd say worth the drive on a beautiful fall day; the festival itself will only take you a few hours so be sure to plan for other sightseeing in the area - there is SO MUCH to do/see. If you do go, pls post back so we can see how the festival has grown/changed and what your experience was like!?