We went to one years ago. It was set up as a surprise birthday party for the groom... it was going to be a surprise wedding for him too (!) but at the last minute his brothers decided he should know.

We were told about it because we lived away & had to organise babysitters & travel, but a lot of people who lived closer & didn't know didn't show. I think like Ness said it was kind of a case of, well you were invited, if you cared you would have come.
TBH I don't know if that's fair, I think it's pretty standard that most people would make more effort for a wedding than a birthday, being that a wedding is one time only and birthdays are every year.

I think setting it up as a pre-elopement party or an engagement party is a good idea, I think you would get a better turn up.