I've shot/been to a few in the past year.
They both have been good. Things I've noticed compare to a venue.
The floor is not level, and you need extra grounds outside for the guests to mingle.
You need to provide toilets facilities.
You need to hire caterers.
You need to get a bar too, and may be a DJ but not a necessity, one wedding they basically had a playlist from their iPod for the night with some rented lights and speakers.
The atmosphere is as you and the guests make it but overall is good.
The downside, is that it might cost as much as a venue. Renting a marquee is expensive, then adding the rest of the cost above, and even renting he grounds for the marquee itself.