I got mine from gotvapes yesterday. I ordered the reg and XL 510 and a pack for the 808's all in 2.6 - 2.8 ohms. I've only tried the reg 510 so far.
The little card that comes with each pack says to remove the plug and inner silicone ring. The plug came out easy but the ring didn't want to come out. I was using tweezers and felt like it was going to break the plastic if I was to try anymore. You do not need to remove ring to fill, I opted to leave it until a video comes out
The gel ring has a couple holes in it for filling. Hard to spot but they are there. I have a 3ml syringe that fit the blue filler tip that is supplied perfectly. However, if your juice is on the thicker side don't try to draw the liquid into the syringe with this tip because it is very tiny and has a hard time drawing. I took the tip off, loaded the syringe with my juice and put the tip back on. Stick the tip into one of the provided holes in the gel ring and fill slowly. I found it easier to make sure the carto was vertical when filling so the air bubbles escape.
IMO - they work very well compared to other carto's with filler. They don't leak, hold a good amount of juice but the juice flavor is lost a bit as well as TH. You will also get a bit of burnt taste if you chain vape and let the carto get hot. The coil is at the top so it helps to tip upside down in between vapes to saturate the wick.
Pace your vaping intervals to let the carto re-wick, use a stronger flavor juice and hold horizontal when vaping like smoking a reg cig. for best results.
Overall - I am happy with them - using a VGO900.