I bought two of those batteries plus an external charger for $21 on ebay last year. They work great for about 2 months and then performance severely deteriorates. I could easily get 36 hours of use out of one charge and (if memory serves me correctly) I think I grazed 60 hours once just being conservative with my usage.
They initially provided way better battery life over the stock, noticably better over the HTC 2100ma, but not as good as the Seidio 3500ma battery. If comparing between the 2100 and 3500, I would venture to guess the actual rating would be roughly 2600ma.
I haven't used them much recently, but the last couple times I used them (while my seidio 3500 was charging) I could go 20 hours and be at 15% battery left. Still better than stock, but quite a bit more bulk for only slightly better life.
One thing I do not like about these batteries are that they do not report the actual battery temperature to the phone. Regardless of how hot or cold it is, it reads something like 78F, even when its warm to the touch. I definitely would not use them in my phone if I needed it for a GPS in a car dock on a warm day. My stock and Seidio batteries can easily overheat (120F+) if in direct sunlight, but at least it will stop charging until it cools down. These batteries would probably let the phone burn up, so be careful.
tl;dr: If you need a cheap/quick fix for battery life but cant afford a seidio 3500 and dont mind the bulk, pick one of these up. However, keep out of direct sunlight and watch for overheating.