I've got a solar kiln and I have had good results with it. I say good, and not great, awesome, or fantastic only because I framed the thing and never really got beyond that. I am sure if I insulated the thing, painted the inside black and installed circulating fans it would dry wood much quicker.
As it stands now it gets quite hot in the summer when the sun hits it,even as drafty as it is. I have never noticed any boards to adversely warp from it. In about 3 months time I can get wood fresh off the sawmill to dry. Mine is a bit small, at 4 feet wide, and 16 feet long and can hold a few hundred feet of wood. Its also wired for lights and outlets. I would certainly recommend anyone to build one.
The good thing is, you can not over-dry wood in the thing. The temp does not get so high that it can honeycomb, or dry out your wood too fast. At the same time you can use a solar kiln for wood storage, since the heating side of the thing relaxes at night, giving the wood a rest period from drying.
Still there are improvements I would like to make and here are some of those ideas. The first is to insulate it the living crap out of it, install some circulating fans and paint the inside black. Then I would like to install some baffles and install a small dehumidifier I have kicking around, to jump start the drying process if need be. Just a flip of the switch and you would be drying...even at night. Some temp gauges running to my shop would also be nice, along with a remote moisture meter too so I could keep a better eye on the dryness of the wood.
One other option I thought of, was to heat water via a compost pile, then pump the heated water through baseboard heat radiators into my solar kiln via slow circulation. By doing this I could raise the temp of my solar kiln, and dry wood even in the winter, or on cloudy days.
Here is a picture of what I have now anyway, but as I said I never got much beyond the framing stage. I've just had other things to build that were more important...like a house