Its pretty hard to go wrong with a solar kiln. The best possible scenerio is that you you get a lot of lumber dried within about 45 days (I live in Maine) and life as a woodworker cannot get much better. Now keep in mind, because of the way a solar kiln works, you cannot over-dry wood with one of these things. That means even if the solar kiln is completely ineffecient, you have a great storage for your lumber needs. If its super effecient you get dried lumber and have great storage for your lumber needs. The point is, the two can be one!!
Now mine is pretty small. The angle of your roof should roughly match your latitude. Mine is a bit steeper than it should be, but still works. There are a slew of design details, too many to list in a post like this, but to answer your question, yes it is a very effecient way to dry wood. At the same time I FEEL anyway, that it is well worth building. I say that though because I have my own sawmill and can esily keep the kiln/ lumber storage filled to capacity for my woodworking needs.
I will leave you with a link to some air drying information on my website, and a picture of my solar kiln:
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"