What Anne said. Works fine, but don't expect it to be a nuclear power plant. At the least, it's good to precharge using the wall socket and then have a backup.
When riding a bike, I've put the solio (the original style, don't have the new one) open on my rear rack and just let it sit there all day while riding. At the end of the day, it's got just enough to recharge my Garmin or my iPod, just barely. If there was a way of taking more time to point it at the sun while riding, it would work better.

I also have a cheap AA charger that I use for my Garmin. Cost me ~$14 and all it does is hold 3 (I think) AA batteries and plugs into the USB port of anything. I use rechargeable batteries in it.
I like the solio, and the concept of the solio, but it's exactly as Anne described. I end up using the cheap AA charger much more often for the exact same things.
I can't find a link for what I go, only how to make them yourself
http://www.instructables.com/id/Mint...d-USB-charger/
But, that might not be a great gift to give!