I've been reading this thread from the very beginning and have learned a lot. Been especially interested in the gunking issues. Decided to try something for myself just to see what happened. Last Thursday I took a freshly cleaned, well broken in, r4 shortie (purchased from Volcano ecigs) and have been using that carto only, just refilling when it starts tasting dry. I'm using two of my home grown juices which are both very clear. Juice A is 70/30 PG/VG McCormicks artificial vanilla, juice B is 50/50 PG/VG Loranns marshmello, both 5mg nic, both with a few drops of stevia extract and a few drops of 151 rum. I've been alternating marshmello and vanilla on each refill and using 280mah 510 batteries the whole time. I'm using only the middle ring installed flat side down and use a 910 whistle tip cut back for a mouthpiece. I've been going through 4+ml of juice a day.
Tonight I took a flashlight so I could see in there better and there is some visible gunk on the coil and the wick at the edges of the cup is looking brown, but it is still tasting just fine. It seems to be getting a little warmer then it was at first, but only when I've been drawing on it continuously. I don't notice it drawing any harder or getting dry any quicker or any other real negative, it's still working just fine.
My take on this is that with the juices that I'm using, one could rotate two CE2 r4 cartos and only need to clean about once a week. In four days of use I've put between 15 and 18 ml of juice through this carto and its still plugging away.