I find my best conference experiences aren't the sessions themselves, but happen in the hallways or at optional side-events. Two conferences I've been to had skydiving as an optional activity, and I've found that's been a great way to make new friends & long lasting connections. Probably something about sharing a near-death experience together.

Most profitable experience was complaining in the hallway to another attendee about the previous speaker. We bonded over our griping & they became a client of mine a couple of years later (even though that wasn't an intention I'd had at the time).

Worst experience was a conference whose topics changed after I bought the tickets, flying halfway around the world for it, then struggling to stay awake for the final sessions of the day. I actually skipped the next conference day, but the attendees were great people & became good friends, so I still got my money's worth.

[Gratuitous plug: I maintain a list of conferences for indie developers, digital nomads and other bootstrapper/DIY types at http://indieconference.com/ I mainly use it as a list of justifiable excuses for travel.]