I had a Mugen Power extended battery for my HTC HD2 and couldn't live without it.
When I considered one for my X2, I looked at the ones offered by Seidio, Yoobao, Gorilla Gadgets, Mugen Power and generic eBay.
Yoobao and Gorilla Gadget reviews were the most positive; experiences with Seidio and the generic eBay batteries were all over the board and the Mugen Power batteries were just too expensive.
I ended up purchasing a Yoobao battery (which comes with a grey battery cover) - that does not match the phone - a plug-in charger and a rubberized case which fits the X2 with the extended battery/cover in place. Total tab with priority shipping was somewhere between $40 and $50, if memory serves.
I've been using this combo for about a month now and am generally pleased. Since I'm rarely on the same ROM for more than a week, battery life is hard to consistently rate. Today, for example, I'm going on 7 hours of steady texting, email syncing, alternating 3g/wifi use, 'net surfing, tinkering with Root Explorer/Text Editor, and playing music on PowerAMP -- remaining battery power is at 54%.
All the extended batteries in this class make the phone huge. Make no mistake, the photos you see are spot on. Some people like the increased purchase on the phone, some people bemoan the loss of that sleek, sexy chassis. It's a personal decision, to be sure.
At any rate, you'll need to calibrate your battery reader to get anything close to an accurate battery power reading. If you aren't rooted, I don't know how you would do this. If you are, you can simply erase batterystats.bin in your system folder, go into bootstrap recovery or use an app that does it for you.
I also alternate between more standard-sized BH5X and BH6X batteries. With these, I need to carry a spare, which invariably gets used sometime at the midpoint of the day. The greatest upside with these batteries, obviously, is in the case selection. I have 3 cases that I rotate, as my mood sees fit. For power users, this is a viable alternative to carrying around a slump block.