My team snagged tickets to the web summit in Dublin last year, not to present but just as attendees. We actually had a great time at it, got to see some good talks, some not so good, and met a huge amount of really cool and interesting people at the eating/drinking events held afterward.
I can see how there would be a huge amount of opportunities there for startups. There is a vast amount of money people, lots of technical talent and lots of ideas all brought together for a few days and I think that if you're socially savvy you could make a lot from it.
What I did not like is how the organisers conduct themselves. Their approach to getting speakers is, I believe, totally offensive. I understand that they're running a business but they spam and bait-and-switch like no company I can recall in recent memory. I believe, or read somewhere, that their hiring practices for their volunteers/staff are pretty awful also.
I suppose it is what you make of it. Would I go to it again as an attendee? If the tickets were given to me and I was in or around Dublin at the time, certainly. Would I pay to attend myself? Probably not. If I were running a startup and wanted to get it in front of people, and to meet people who might be interested in it? I'd probably pay €1500 for the opportunity. It's not very expensive compared to the opportunities it may present. Worst case scenario is you have a few fun days in an interesting city.