Do many of you use the spinerval series for time trial training? I got the Totally Time trial DVD and I like the motivation it provides, but it was pretty tough. (4 sets 15 min each at LT with 2 min rest between) I'm sure it'll get better as I get stronger. How many times a week should I do a really hard workout like this?

They are great workouts. I would only do one per week. A long ride would be a second hard day.