I would advise boiling any carto or atty before using. Even if they have no filler there is usally some residual flux or something else in them. I'm way too sensitive to the foul taste that comes off of any so I just make it a regular habit to boil them first. When I boiled a couple yesterday there were three little back slivers that came out of them. One looked like a splinter and was about .5 cm in length.
The first E2 r4s I (and many) got were terrible and extremely inconsistent in their wicks length and nuber of wicks. One came oozing filler. Four of the regulars had tea colored, stiff, rodent urine smelling wicks. (I took them all apart.) This was when they first came out and several vendors were receiving bad E2s from China. Not their fault as the packages are sealed and hard to inspect. With new arrivals coming in the ones I received from COV were the true rev 4s but I still get residue taste if I didn't boil them.
The E2 r4s are still pretty inconsistent especially in length of the wicks that hang into the juice tube. Lots of people are working on mods for them to improve them. One thing I have done to improve wicking is first to boil them then leave out the bottom ceramic cup ring/sealer and slide the middle ring down to the cup. This doesn't keep the juice tube as sealed but I'm getting no dry wicks. In fact when I take the drip tip off there is juice on top of the ring around the edges and filling the cup but it does not cause it to act flooded. One of my C2s actually has a layer all the way across the ring with juice floating over it. Same results. When the reservoir starts getting empty and the wicks start drying I don't get a burnt taste due to the small amount of juice "floating". I'll get a more airy draw and then know that I need to refill. When I get the airy draw I can still get a few drags because of the juice that is floating on the ring.
I haven't had any issues with leaking but I don't turn it upside down and shake it. I doubt that the juice would leak out of it anyway if the juice is at least 50vg as are all of my juices I use in the E2. I do lay the mods with E2s down on their sides, though, and have no leaking issues.
I have the reg and XL, both in 2.x ohm and 3 ohm. So far there is not enough difference in the two to make a good decision on which to use on a 3.7. I haven't tried a whole lot of juices in them. And most of our juices have high percentage of VG. I haven't tried any 100% PG with them yet.
Taking out the cup ring also makes filling much easier and you can just drip the juice down the side instead of needing a syringe.
Hope this helps anyone.
Alice