Slim Series are a nice change; I don't find them as intense as cathe but they are not easy. You've got to love a lower body workout with no weights and NO LUNGES!!! Also, I found that while I didn't notice any losses in overall strength after doing no cathe and all Slim Series for 5-6 weeks, I did notice loss of strength specifically in my forearms. I think heavy weight training (even such as that in PH) has a secondary effect of keeping the forearms strong.
I also notice that I don't have the same stamina to do big girl pushups anymore. I could never keep up with cathe but I was getting pretty good (I could do probably 20 in a row before) but there are really not alot of pushups in slim series and I lost the knack a bit.
While I was doing slim series, tho, my hands were always sore, so who knows? maybe now that I'm back on a Cathe rotation I'm losing hand strength!!
Christine