I haven't researched any of the replacement batteries in any real detail for one simple reason - the 3rd party batteries often underperform to such an extent that I don't care to invest any money in them. I'm skeptical of their claims and suitability.

LiIon or LiPolymer batteries have a charge that's based on their size, so to double the capacity of the battery you have to double the form factor. If the batteries that are out there are smaller than that, then I question the capacity claims, I question whether the shielding is adequate, and whether I should be using one.

The posts I've seen in other forums tend to reinforce those concerns. So I've simply purchased a spare standard battery that I keep with me in my bag or pocket (it's small enough for that) for those times when I might need more juice.

But there are posts at XDA-Developers on various batteries, most of them les than stellar.