I apologize if this has been covered, I found posts mentioning it as an option, but not much on reviews.
I recently ordered/received a Motorola P790, portable battery/charger, from ebay along with a mini to micro USB adapter and wanted to pass along my initial review. I will admit that I was hesitant to purchase it based on limited reviews, but figured for $10 shipped for both, worst case I would have another paperweight.
So far the battery has performed well. I have used it multiple times on my X, each time when the remaining power level on the phone was less than 30%, and the Motorola P790 was able to recharge the phone's battery within 2 hours. Each time the indicator on the portable battery was flashing orange, which indicates that the charge is low (don't have the manual with me, but I thought flashing orange was between 30-50% remaining), but I have not tried to charge the X further before recharging the portable battery. I will have to try this, but my concern was whether I could get at least a full charge out of the battery.
I have also used it while watching a movie, and was able to watch the movie with no discharge to the X's battery. Without the P790, this was a major drain on the phones battery. Again, I am not sure how long the portable battery would last while watching movies, but since it is a 1700 mah battery, it should provide enough juice to prevent the phones battery from draining for 2-3 hours of movies while on a flight or other situation where a charging source is not available
The size of the P790 (approx 2"x3"x1") makes it convenient to throw in my carry-on luggage, but depending on the situation could be easily carried in a pocket as well. The battery will also charge any other devices that use mini USB (i.e. mp3 players, bluetooth headsets, etc.) Another nice feature (which I haven't tried yet) is that you can charge the portable battery and phone simultaneously, if this works it will cut down on the number of charging cables I need (as the P790 needs a mini USB cable to charge and most phones now need micro usb).
The only negative that I have so far is the fact that the portable battery connector is a mini usb, the mini to micro adapter solves the connection issue but means that I have to carry the adapter as well. I did make sure that I purchased a Motorola adapter, but a generic should work as well.
Hope this helps anyone in the market for a portable battery/charger. I know it has saved me a couple of times while traveling so far, and I have only had it a couple of weeks. If I find any other negatives, I will try to update this review.