I like the program except.....
I'm not a master typist, and the timed tests are too fast for me. Sometimes I find myself getting something incorrect because I'm in too big of a hurry trying not to have the clock run out, either by being fumble fingered in not being able to type fast enough, or not reading all of the choices in the multiple choice categories. I seem to know all of the items in the tests, I just can't do them fast enough and I am finding this to be less enjoyable because of it. If there were a program just like this without those time tests, I would use it in a heartbeat.
Maybe i'm pointing out the obvious but you absolutely don't have to use the timed practice. Even if you hit the "Practice weakest words" link - the only part of the site where you get timed questions as far as i know - you go to select "practice without a timer" on the next page.
I think they may be testing out different exercises on different groups of users, but some of us get timed exercises as part of lessons, where the introduction page says "Timed bonus round - Finish in less than 2 minutes!" and it's the same questions as regular lessons where words are introduced, only there's a timer counting down. If it runs out before you're done you still get to finish the exercise, but you don't get as many points.
So you do have to do the timed lessons if you want to "master" a unit.
I haven't had any problem with this issue so far, but then again I haven't taken the timed challenge more than a couple of times. I wouldn't mind an option to configure the timeout, perhaps a difficulty setting of sorts, asking more questions or handing out less points in exchange for a slower game.