Yes, you can refill them. Push a straightened paperclip through the hole in the mouthpiece and the cart will pop out. The blu carts are sealed with foil. I take this off so there's no chance it will get stuck in the atty. Drip a few drops, and pop it back in!
The exposed atomizer wick makes it easy to dip or drip as well (do a forum search for dipping and dripping for more on this.)
As far as
juice goes, it's really going to be a matter of personal taste. I can say that Totally Wicked had great service and tasty
juice, and the couple of Johnson Creek sample pack flavors have been tasty as well though they took FOREVER to ship. (I LOVE the Peach Cobbler... it tastes like Peach Snapple to me) I haven't tried the juice I got from Ruyandirect yet.
You can also make your own juice! Check out the DIY E-liquids part of the forum. YUM!