Extra battery for Sony Alpha - which one?

I would like an extra battery for my Sony A700 and wonder if I should only use the Sony brand battery, or if the generic brands are okay. The other 3 I see, which are less expensive, are:

1. MaximalPower
2. Synergy Quality
3. High Capacity InfoLithium

If the generic batteries are just as good as the Sony and won't harm my camera, I would rather use one of them instead.

Thanks for any advice you might have to offer. :)

I purchased two batteries on EBAY. Don't remember which seller, or how much they were, but a lot less than Sony. I have not had any problems with them. I am using A200 cameras

Phil