As with any third party launcher, it runs on top of the stock one, so uses a bit more memory and therefore power than just using the stock one.
A top tip is to make sure you remove all widgets, live wallpapers etc from your stock launcher homescreens before using the third party launcher and even do as I have done and remove all but one homescreen from touchwiz by pressing menu/edit and deleting the un-needed screens.
that way the phone has less to store in memory in the background.