I've recently got into Li-ion cells and my first batch are all Pila. The only problem I had was with the fit of the 17670 (Pila No.600S) in an E2e body with an older KL4 head, which is effectively an L4. When you insert that size cell it wedges itself about 8-10mm shy of the bottom of the battery tube so you cannot get the head on and the tailcap spring won't reach the cell. This is due to the inside of the tube tapering at the end. Although I've only tried this with Pila as that's all I have, other people have reported the same problem with the E2e body and also the L4 with various brands of 17670 cells. However, this does not mean that the E2e/L4 are totally incompatible. Because there seems to be more tolerance variation in the E series parts, plenty of people also report that they don't have this problem. If you do have this issue, it is possible to shave off a small enough amount of metal from inside the last 10mm of the battery tube to allow a 17670 to fit. I did it only this weekend with a Dremmel through the tail cap end and it works. Others have used a dowel with progressively finer grit sand/emery paper wrapped around and inserted into the battery end of the tube. Either method will work but takes time and patience. The trick is to take off only a little metal at a time and to periodically test fit the cell. You want to enlarge the tube just enough to take the cell but not so much that it rattles because if you then put in primaries there will be too much play. I would also advise that you aim to make the tube surface as smooth as you can (hence the ever finer grit) so the tube doesn't scrape and gouge the cell. Make sure you work evenly all round the internal circumference otherwise you'll end up with it lopsided, and don't forget to clean out all the grit and metal particles before use. As far as the finished mod goes, the 17670 should then work perfectly, but I did notice that the KL4 head does heat up a lot quicker than on primaries and I was a bit concerned about how the Li-ion cell would react to the body getting so hot.
Another method I've read about is stripping the thick wrapper from the cell and giving it a thinner tape wrapper. Apparently it works. Personally I wouldn't do it because although it's quick, I don't like to monkey with the cell itself and it means you may have to do it with every Li-ion you put in there, whereas modding the tube means you only do it once.