I have an 1000mah JoyeTech eGo-C Upgrade. I didn't know that's what it was until after I bought it, I thought it was a standard eGo battery. Anyway, I have a feeling the idea behind it was "pre-Twist." You can find vendors advertising it as a variable voltage, but it's really not if you read the fine print. It has two modes: A constant mode, where the voltage stays at 3.3v; and an unregulated mode, where fully charged it puts out around 4v, but the voltage goes down as the battery discharges.
I guess that
technically it's a variable voltage, since the voltage changes, but you don't have much control over it. It works just fine as a battery, but clicking back and forth to different modes with no clear indication of what the voltage is seems kind of pointless. I also haven't noticed that big a difference, so I just leave it regulated. I still use it, but I use my Twists more often. YMMV