Eh, the overflow cup is pretty much a useless thing I have to remember to clean once in a while, but the side fill valve is great assuming you can find the syringe tips that fit it. I absolutely hate unmounting a tank from my mod when I'm out of the house, and top fill is just as irritating for only getting a half-tank full.
The trick to the 3.1 fill valve is to follow the instructions on not using a needle tip bottle, you rend the valve spring out of alignment or cut the little O-ring pretty easily.
I've got two of the Ehpro KFL now. Same build, really, just better heads on the screws than on the Hcigar 3.1. But those are easy enough to replace. Airflow on the Lite is considerably tighter than the 3.1 will the screw removed, unless you leave a gap between your mod and atty and pull the screw entirely. Otherwise, leave it in, the screw is short enough that when backed out flush with the base, the airflow is as open as it gets, and it'll help keep minor post-flood drip from seeping down into your 510.