The therapist definitely has the power in the relationship once the client attaches in some way, but the client can (depending on how the therapist sets it up) have equal or greater power in choosing what is discussed and when, and certain choices about method. At some point, though, if it's a therapist that's heavily leaning toward a certain approach--say behavioral—and the client is in disagreement, there will not be any give on the therapist's end. The client either leaves or submits.

If there were not a power imbalance, then there would be no need to have protections in place for the client. The varying levels of imbalance depend on how skilled and ethical the therapist is. I could see some clients feeling equal, and others being totally effed over with mind games and power plays, and everything in between.