I note that very few of the comments above display any real knowledge of WebSummit (except Assaf) and they all talk about conferences in general which is not very relevant. They are not all the same.
The history of WebSummit is that it started in Dublin (Ireland) and has this year moved to Lisbon. Collision is a sister event.
Its not in any way an industry specific event (other than its a technology event) but it does attract a lot of big names and big VCs.
So if you are looking to find customers its probably a bad way to spend your money. On the other hand if you are willing to "work" the conference you can make great contacts especially around the VC and partner area.
As one of the other contributors said, you should find out beforehand who is attending and arrange to meet them during the conference. You will get out of it what you put into it, as with most things in life.
I was involved with one Company that chose WebSummit for its official launch a couple of years ago.We got an astonishing amount of interest. However due to our own incompetence we failed to probably follow up properly.
There is an incredible buzz to WebSummit, and there will be a huge and very international attendance in Lisbon this year.