I play Vipers, and I'm pretty much to the point of being obsessed with them. I do not like SG's at all, yet I find the Viper visually appealling.
IMO it's a little bit of a mistake to compare the Viper to the SG. While the shapes are similar, the body thickness is not. the extra thickness and heft of the Viper puts it more into LP territory IMO.
I don't find them to be neck heavy at all, but I use Kepur straps and they stay put. I prefer the strap button on the back of the upper horn myself.
I have 2 LTD 301's and a Max Cavalera LTD signature and 3 late 90's ESP Vipers. The ESP's are all bolt-ons, which I prefer, and have a little bit different sound. A little more attack and snap, but are also more resonant and sustain a little better than my LTD's. I'm a fanboy of late 90's ESP's in general, as their standard bolt-ons IMO are killer for the price. My Vipers are solid 1 piece mahogany bodies with 1 piece maple necks for the most part (the little "wings" on the headstock are seperate glued pieces).
I'm a fatass, and the body fits me a little more comfortably than a lot of other guitars. Superstrats and SG's look like toys, as do most singlecut shapes. the extra width of the Viper at the horns and the stockier body look better and the playability for me is spot on.
here's some vids of mine in action:
4:35