Aye, my very first one the battle lasted 4 minutes, and I got a total of 7870 exp.
You don't even seem to have to do anything to get exp at all, if other people kill a mob, even if you didn't so much as touch it, you get exp. The weakest mobs (detectors, ~2k HP) give like, 1200-2k exp. It's seriously ridiculously fast exp. The battle may only last 4-5 minutes for now, but they happen fairly often, and as Zman says, they'll probably last longer once we get the resources up.
You also get 20% of your exp in cruor, and 10% of your exp in Resistance Credits (or the other way round maybe). After 2 4-5 minute battles, I had enough to buy both my AF legs (1k credits each) as well as some temp items (Lucid potion/ether III, lucid elixir II, Mana Mist (gives 300MP to EVERYONE in range, not just party members). These items stay with you just like they do with other abyssea temp items, they're not a bastion only item.
All in all, this event kicks arse. There don't seem to be any items from them, but I think we only managed to repel one attack while I was there. It doesn't look like there will be any items though regardless.