I have used all of the products you mention and have some thoughts.
Volt - This is my preferred product. I have the mega PCC with 65mm batts. It will hold 3 carts plus 1 cart on the batt, all in a single case that last me all day. Customer service is great and prices are good too.
Bloog - Almost identical products as the Volt (I am convinced they are made in the same factory) but costs more. No reason to pay $2 more dollars per box of cartos when i can get volt stuff.
V2 - One of my first products, just like bloog and volt, they are 808. But the prices are high and the products are subpar to volt/bloog in my opinion. Batteries are unregulated.
Eroll- This is a cool cig. I just got mine and am using it as we speak. The product is a little finality though. Sometime you have to fiddle with it to get it working right. The biggest disappointment is the size of the tanks. Its a whopping 0.4 ml and I blow thru a tank in 20 puffs. Also filling the tanks are a real pain in the rear unless you have small tip bottle or syringe. All that said its still a really cool product, i have a love hate relationship with it. today i used it exclusivity and burned thru 7 tanks.