I'm using one right now.
The Inferno comes in two sizes - 650 USB can be used as a pass
through or regular. It lasts me most of the day on a charge.
The 1000 is slightly longer, and would last me easily all day.
Volcano is about to come out with a
tank system - it may be in the store tomorrow (shipping problems behind the scenes). Their forum will have the details on this.
The inferno will accept the cartriges and attys from the magma series (I believe these are the same as any 501 model), as well as cartomizors and clearomizors. They make for an outstanding drip machine. They are well made, and if you should have problems with them, Volcano has an excellent customer service department.
You can order them as a kit or the pieces separately. If you order separately, be sure to order the cone (although it is for cosmetics only, it kinda looks bad without it) and a charger. Details are on the order page.