Yes, I started MS2 6 months before my step 1; Even if I just use it for MS2 material such as path, pathophys, pharm, micro its still useful as all crap. To be honest, what was really covered in MS1 isn't so hard to review. Think about it, Histology, Biochem, Anatomy, Neuro, Immuno, Physio, Behavioral. The only difficult, voluminous subjects would be anatomy, neuro, biochem. You revisit, physio/immuno/biochem a bunch in pathophys and path. If you have ICM classes like mine, they love to test behavioral. So really MS1 is a joke when it comes to reviewing compared to keeping up with MS2 which is the majority of Step 1.
If you want to give it a go, send me a PM with your email and I'll send you a referral link which will give you 1 month on top of your free month you get for trial if you buy. Give it a try, its helped me tremendously going through it while doing my MS2 coursework.