Are you the type that's always desperately searching through a convention/conference schedule? Hopping from a panel to a screening in search of a free seat or more exciting content? Addicted to your smart phone and wanting to apply it to ever more aspects of your life? Then I heartily recommend the Guidebook Android app. It may also be available for the iPhone; I haven't checked.
I'll be attending Japanese anime/manga/culture convention Otakon this weekend up in Baltimore and downloaded the app after hearing about it from Omar. It has exceeded my expectations: the schedule is searchable, you can easily add and remove events from your own personal schedule, and it even offers reminders. However, the best part is that it seems both fast and robust. Even the slightly clunky bits, like the listings for fan parodies and AMVs which presumably had to be updated at the last minute, aren't really a problem.
While I'm writing this up in the context of Otakon, I think I'd find it handy for professional conferences as well.
Feature for which I'd be willing to buy a premium version: The main additional feature I would like would be the ability to share schedules and maybe check-in to events. I'd put my price point for such a feature at $5 for a premium app or the like. I some how doubt that there's enough demand to make it worthwhile, so for now I'll appreciate the features that are there and try to remember to "like" my favorite panels as I go.