Depending on how you want to look at it...
The advantage at any given moment depends on the count, just like always.
The penetration of doubledeck can be fairly easily estimated, just look at how many cards are in the discard tray when the cut card comes out. 1 deck out is 50%, 1.5 deck out is 75%, and then you can fudge those numbers around.
If you're looking at the overall desirability of the game, then you'll probably need to piggyback on others math. Either buy Blackjack Attack, or run some sims.
(50% is quite poor, 75% is good, many games seem to be sorta in the middle)