When the Swarovski 8.5x42 EL was first released, the company had a rebate on the Swarovski doubler such that it only cost $100. I went ahead and got one at the time, but I wouldn't purchase one now given its high cost and marginal utility. Does mine work? Yes--though the field of view is narrow, it is of high quality, and significantly increases my ability to discern fine details. Over the years, it has allowed me to identify a few birds of very high interest that I otherwise would have had to forfeit. Is it convenient? No, from the standpoint that it takes a while to unscrew the eyecup and thread on the booster, and yes from the standpoint that it can be carried along at times when I have decided against bringing along a scope. It is a tool to be used in the same vein as the best pocket binos--only at times when you wouldn't choose to have more appropriate equipment along. With the doubler attached, the bino is effectively converted into 8.5x and 17x monoculars--it is easy to switch your eye from one view to the other (my dominant/master eye is my left eye, so I screw the doubler onto the left side, which allows me to use my left eye to either view through the doubler or through the unaltered right side of the bino). I find that if I brace against a tree or rest my elbows on my knees while sitting, that the view is quite steady and no tripod is necessary. If I take the trouble to carry a tripod, as I often do, I use my scope (more powerful and comfortable) not the doubler.
--AP