I use Transcender purely because I bought a training package that gave me access to the practice tests for 180 days or something like that. I haven't actually taken an exam yet, but I don't love the Transcender format. It's clunky, the questions are badly phrased, the interface is not intuitive... I wouldn't spend my own money on it, put it that way.

That's just my opinion though. I'd be interested to see what others think.