They have free accounts that you can try out and you get a lot of the data and basic data management with that.

You need a premium account to get all the WKO+ style power features, performance management charts, etc. It's a bit expensive per year if you aren't using it with a coach, IMO. One cool thing about that, though - they have the whole workflow for user -> TrainingPeaks -> WKO+ -> coach worked out, so either the user, or the coach, or both can use either training peaks.com or WKO+ to manage their data/athletes.