I think that if you are going to use eDrive it may be better to use it on an S2 or S3.  I see that you can use the eDrive on the S but am thinking that there could be more accuracy on the new model S's.  If you already have an S or can get it cheap enough and depending on what you want it to do - it probably would work fine.

I have an S and S3 and erive that I use on teh S.  I should have tried the S with the eDrive to see how well it works but never took the time this spring.