The Silent Partner is great...if you don't mind the $225 price tag. I have fallen on it a few times aid climbing, and it locked up quickly and safely. I have used it in wet weather (soloed Green Dragon a few weeks ago) and it didn't seem to slip when I took a relatively long fall, even with a wet rope. The Silent Partner doesn't seem that great for free climbing. There is a loop dangling below you, assuming you tie a back up knot, that seems to get caught on stuff as you climb.
The best thing a soloist can do is find a "real" partner.