As a new convert to the double hander, I'll take a kick at fielding this one. As background, I've been a fishing addict since diapers - according to my parents, toddling around the yard playing with sticks which I dubbed "fishing robs" and nearly drowning myself a few times crawling into the water trying to get at minnows which I could either see or imagined. In the 35 odd years since my love of the sport has ebbed and flowed, but never gone away and I've been lucky enough to fish throughout BC as well as a few other places around North America and even worked my way through university as a guide at a fancy fly-in saltwater salmon resorts off our west coast. Over the years I've become far less concerned about catching fish vs. getting out on the water to enjoy the whole experience, and for the last 10 years have pretty much been dedicated to C&R flyfishing vs. all other approaches. In a nutshell, while it is sure nice to catch some fish, these days I am more than satisfied if I can just spend a nice day out on the water away from my blackberry, enjoying casting, using some flies that I have tied and exploring a beautiful place. I think this is an experience that is somehow much more typical of flyfishing in general than other forms of fishing.
Last summer, for the first time I had the opportunity to cast a friends spey rod and it was love at first flail. I have not looked back since and for me, the double hander has firmly lodged itself as my absolutely favourite way to fish. For me, the reason for this is simply the complexity and the beauty of double handed cating vs every other way to get a line out there. I can happily spend days whiling away the time playing around with all the different cast styles (and trying each particular situation in which they are needed or advantageous). While this is somewhat possible with a single hander, there is just so much more cool stuff you can do and work on on with the double handed rod. And when you nail a cast just right it feels sooo good. For me, since moving over to the double hander I've just found my time on the river so much more rewarding - the casting alone makes a trip worthwhile, and there is a lifetime worth of learning and perfecting possible. Add to this some nice scenery, comradery and a few fish and I'm in heaven.
Technically, yes the double hander is much more efficient fo covering water all day yada yada... But it beauty and complexity of the casts IMHO that makes it such a wonderfull way to fish.
My $0.02.