It doubles your trigger size, an increase of 100%!!! (note my sarcasm)
Alright, I never used a double trigger before because my old marker was an old Spyder TL w/o the double trig. After about two weeks with the A5, I decided to get the D.T. Set up is simple, take old one out, put new one in - 2 minutes. At first it stuck a little... took it apart again, applied gun oil to the friction points... no more sticking. It works perfectly, no problems. There's not much to say about it though, as it's just a trigger. It didn't really increase my rate of fire as most people claim it does. Although it's nice to switch fingers if one gets tired, but the whole "up-down-up-down" firing method that i thought would rock, didn't really work out. Overall, I think it's a high quality product. It's easy to set up and it functions flawlessly, so I guess it's just a matter of what you prefer, single or double trigger. I could take it or leave it, personally... But since it's designed just fine, is easy to install, works perfectly, and I can't think of anything to improve it.... i'll give it a 10.
I done already gave it, Ma! (note my hick-like accent)
if you mean by "up-down-up-down" walking the trigger (index finger than middle, index than middle, index, middle, index, middle, etc.) and you were dissapointed by it, thats because it requires practice. Nobody can pull the trigger 20 BPS with walking their first time walking the trigger! It takes practice. But with practice, I've seen over 23 BPS.