My thoughts on a grip.
Wanna look like a Pro? get one.
Want to make it easier to shoot verticals? get one.
Apart from that a spare battery will do the same thing.
I bought a cheap imitation grip from you know where and from a well known company. Had all sorts of problems with it.
They eventually agreed for me to send it back and I paid the difference and got the real thing.
When I switched from my 1D to the 5D I did notice it was harder to do the verticals but what the heck it was only a minor adjustment.
So in summary, if you feel you need one get a genuine one with genuine batteries.
Otherwise put the money into better glass.