There are several advantages that this larger case provides.
1. The cells pack together a bit better than 18650's. This means more total capacity in the same amount of space.
2. The greater cell capacity was significant. I've been using 18650's and LIPO for a good while and really wanted a cell that would have lots of capacity per cell that wasn't LIPO.
To date, I have been testing on my SkyRC D100 charger and a 6S 21700 battery holder I made.
Cell specs say full charge at 4.2 volts and discharged at 2.5 volts. Everything I've ever known about LION before says fully charged is 4.1 volts and discharged at 2.75 volts. Did something change that I'm not aware of?
I tested the cells soon after their arrival and they were all at 3.58 volts. I then charged them to full (4.1 volts) and recorded their capacities. All cells came in at around 3400 mah. Some had a bit more capacity.
My SkyRC D100 charger doesn't go lower than 2.9 volts and these cells are rated down to 2.5 volts. I'm not really testing them to full capacity when I do a discharge test. My results so far are a bit skewed as a result since I'm not going down to 2.5 volts or up to 4.2 volts. So far I see 3600mah down to 2.9 volts. They are rated at 4800mah so that means that between 2.5 volts and 2.9 volts and above 4.1 volts is another 1200mah that so far I am not measuring. I'm skeptical, but we will see. My next round of testing will be at 4.2 volts.
My experience with any battery pack is that the closer to discharged you get the more sag you experience. My expectation is that these cells at below 2.9 volts are going to sag a good bit. My D100 discharges at 400mA which is not a load test, just a capacity test. I have 35 halogen bulbs I've used for load testing many times. Each bulb is 12v, 55w, 4.6 amps. 2 of these bulbs in series will be pretty close to a 1C load on the 6S pack. I'll put a watt meter in between the pack and the load to measure Ah and a cell monitor on the balance lead to measure individual cells.
I have checked 12-15 cells to determine their Ir. Most are in the 35-38 milli ohm range and a few as high as 43 milli ohms. This seems somewhat disappointing, but everything I've read elsewhere shows this is a typical Ir for this cell. However, it may be that these cells don't increase their Ir like most cells do as they age. In that case 35-38 milli ohms isn't that bad. Very rarely do people scrutinize their packs individual cells after they are 6 months old or older. It is common for cells to creep up to 40 milli ohms or higher in that time frame.
Future LION development is coming. Supposedly 6000mah capacity in the 21700 is coming soon. Maybe wait until we see capacities over 5000mah before buying? I ran into a deal on what are supposedly Samsung cells rewrapped to be generic and are rated at 5000mah. I didn't buy them. If Samsung won't at least put their name on the wrapper, why should I trust them?