Welcome to the forum.
Flavor:
That is all dependent on how much flavoring is put into the e-liquid. Yes refilling the cartomizers can improve the flavor and or change the flavor. I find all e-liquid has an after taste. Some I like and some I do not. the ones I don't like I don't vape anymore or I mix them with a flavor I like so that liquid doesn't go to waist. But in the end they all have an aftertaste. Not sure if real tobacco had an aftertaste and if it did I was so use to it I didn't notice it. The one thing I did notice was the smell of tobacco on other peoples breath. That is one reason I wanted to quit analogs as I knew I had that same smelly breath.
VG promotes better vapor production and PG promotes better flavor and throat hit (The throat hit I could care less about). So most e-liquid companies use a blend of PG and VG, usually somewhere around 50/50. But there are companies that use all/only VG and they UP the flavoring content to promote better flavor.
So no good flavor can be had with either all PG, all VG or a blend of the two.
Doing a search for Prosmoke I found their website, it is SLOW. But if they are using the newer style of cartomizers like V2 is using then they are not refillable. They are a sealed unit. But I don't know that for sure. One way to tell is if it has a removable mouth piece, a rubber/plastic stopper on the end you put in your mouth, they are refillable.
If you are looking for something similar that can be refilled, the cartomizers have good flavor out of the box, the price is less and that you can use the gear you already have with it take a look at Halocigs.com.
It looks like that Prosmoke gear uses the same batteries and carto threading as Halo. So everything should be compatible.