I have bought cheaper versions of mobile batteries before and found them not to last as long as the genuine one. I would read the manual or google for info as using another brand battery may void your warranty if any problems. It is recommended that you use same brand.
I bought some cheaper batteries for my Canon camera, worked fine initlally. A year later they wouldn't hold charge or recharge at all.
Its not too bad if you're conscious of it or using them everyday,but i wasn't so when i took mine on vacation to find i had no sodding batteries except the genuine one in the camera. Fortunately i have an adapter which takes pencil batteries so i got it resolved.