I just happened to have bought an eGO 650 (the 900 wasn't available at that time) at the same time that I bought an XPower 900 and 900c (One is an atty and cart system and the other is a cartomizer set). The XPower 900 set came with two cones, two attys, two bats, usb and wall chargers, and 10 carts while the 900c came with same except it had 10 cartomizers. They cost me $47 each fast including shipping (China). I tried all three out for 5 days and my eGO has been in my desk drawer ever since.
The XPower is slightly wider (2ml), and slightly longer that the eGO. It uses a 301 connection with a modified atty, carto, and carts that make them larger and improved from the older smaller ones. Both have more capacity than the standard eGO - with the cartridges being twice as big. Yes, eGO did a knock off to make their new "Mega" parts for the 510 (which are the same size as the standard XPower's)and their new ego Mega parts - which are additional costs - hold more juice.
Comparing the two, the first thing I noticed was how much more vapor I got out of the XPower. I also thought I got a slightly better taste/flavor - but only slightly. Then, being an atty and cart guy, I soon noticed how much longer the cart lasted. I also notice that the wickening material in the XPower seems of better quality - it's not as "cotton ball" as the eGO and will vape wonderfully untill it's virtually dry. I also really liked the slightly larger size of the XPower as it felt much more comfortable in my hand.
I have a friend who is a JOYE eGO fanatic. He bought the 650 Joye ego, then bought the 900 batteries. So for fun, we spent an evening running all kinds of tests, including "blind" tests where one of us was blindfolded and the other held and pushed the button while trying out a variety of juices.
At the end of the evening, he went home and ordered an XPower. He definitely preferred the feel, the larger button, the overall appearence and especially the difference in amount of vapor.
The eGO is a *very* good e-cig. But if you're considering getting an eGO (Joye or whatever), you might want to also at least consider the XPower. (I'm going to put on my hard hat and get under my desk as I know that I will be attacked by the "Joye" boys/gals.