I bought my BT double trigger from Actionvillage.com and it works great. It does exactly what it promises. It changes your trigger to accomadate two fingers instead of one. In my opinion its a ton more comfortable for my hand. This may be because of my previous experience with spyder guns before my BT but w/e. Also it only cost me $16. You can fire a tiny bit faster with the double trigger than the single trigger. Maybe like 4-5 BPS instead of like 3-4 BPS. My only complaint was the lack of installation instructions. I did manage to install it fine by myself, but if you are having problems you could probably call up BT and ask them about it.
Great product that does what it says. If you don't want to go for an e-grip, but you want a double trigger this is really your only option and I think it's great. One point off for the lack of instructions, but not really a big deal. Look at how your regular trigger is installed and you should be able to figure it out.