I've never heard of Law Preview before, but I used LEEWS and I finished 1st in my class this past year.
I can't say how much of the credit should go to LEEWS, but if I had first year to do over again, I would definately still buy it. I did not use LEEWS all year long, but I did spend a couple weekends with it before midterms and finals. It helped me to relieve stress, gain confidence, and understand what professors are looking for when they grade exams. But I also studied my ass off all year with all sorts of aids. Still, for less than $200, LEEWS is a no brainer.