I'm only two weeks into this. I've got three eCigs, and I'm leaning toward my KR808D-1 as being my favorite. I'm aware of the 510's popularity, but I find I'm less drawn to it than the other two ... it's coming in third place. I have had no difficulty refilling the cartomizers. I find a small hex wrench (1-1.5mm) works great for removing the end plugs, and then just drip the juice slowly in. Vaping seems more consistent and flavorful than with the other two eCigs. Get the extra long batteries for battery life ... and be sure to get the PT. I've got auto and manual batteries and prefer the manual. I've got one short battery which was included in the kit (2 wks ago) which appears to be dead, won't charge.
Buying an extra battery essentially gets you an extra eCig for $15.
Can't wait to see if the Juice Box knocks my KR808D-1 off my #1 spot, when I finally get it.