We at Codacy decided to attend the Web Summit last year and invested the amount they asked us to put in (I can't recall how much but was the standard for the BETA category). They upgraded us to the BETA category even though we actually should have been put in the lower ALPHA one.
In any case we executed on a plan: to extract as much as we could from it and that we did:
* We abused the day with the stand to talk to Startups and give awareness of our product. We actually met a few of our best customers.
* We did a list of people we wanted to meet and took some effort to network and reach them. We got into very interesting discussions and follow on meetings after the conference. We met great engineers and people we ended up partnering with.
* We took part in the pitch competition (with some hesitation at first to be honest..), but took it seriously and we ended up winning the whole thing (techcrunch.com/2014/11/06/lisbons-codacy-wins-the-web-summit-pitch-competition/)
So while I cannot speak for these new conferences, for us the Web Summit was certainly a great investment.