I use a Tippmann 98 custom with a few nice upgrades. Nothing special.
*It saves gas and money.
*With a long drop forward it puts the bottle back a bit.
*No one believes me but I think it may do a little bit of anti-siphoning.
*Makes your Co2 bottle look intimidating.
*Saves o-rings. Ive had my on-off valve for a while and I havn't had to change an o-ring yet.
Hmmmmmm. Nothing that I can think of except it may add a gram or two of weight.
This is a great investment and I'm sure Ive saved money by getting one. If you have a marker that sprays gas everywhere when you screw in your tank, then I recommend getting this.