+1 and agree wholeheartedly with DIYHUNT. I own a doubler. I really do not see many pros. However I can only speak to how I used it with my Bushy Elites.
I own Swarovski 10X42 Els as well and here is my .o2
The only positive it had with the Elite doubler is the fact that it could be immediately placed over the lens. However, with the Swaros you have to unscrew the lens and then screw in the booster/doubler. This is time consuming and I do not like the fact that I have to worry about my bino lens out in the open.
Furthermore, the light gathering abilities in dawn and dusk hours are greatly reduced. Also, you will need a tripod for sure (you would with a scope anyways, but....) I find that it is a little harder to acquire objects with the doubler vs the scope. Your FOV also goes down even more making it very limited.
If weight is the concern and you want something that reaches out further, get a Nikon 50 mm ED. It will cost you more than the $400 doubler but not by much if you buy on the web. Even if you get the 13-30 expiece it is not bad. I purchased my scope $399 and eyepiece separately so I could get the 13-40 eyepiece. Although I now own a swaro 65, I will be keeping the Nikon if I need to hump it a long way. The Nikon only weighs 20 oz. And is very clear.
Check out the full skinny here:
http://www.shopwise2000.com/index.asp?
It is by far the lowest price on the web you can buy a quality Nikon. I get nothing from this guy and he was great to get it to me immediately last spring before hunting season. He has nearly 100 in stock and you may be able to talk him down even lower than his already bottomed out $559. They have been sitting and he wants to get rid of em. He was willing to give me an ever greater discount if I bought more than one.
You will pay $700 at the nearest Sportmans and you cannot purchase them from Cabelas any longer. I have not seen anything else on the web lower than $600. Cameron Hanes mentions it a copy of EBJ last yaer and I spent over 20 minutes with Guy Eastman at last years Salt Lake ISE and he convinced me to buy one. He told me how much he uses his if he is not carrying his beastie 80mm Swaro.
Good luck on whatever you decide.