BMWPower,
I think you right on with $50/hr.
I'm also in similar situation where I have garage with lift (not nice as yours) and friends come over to "check what's wrong"
I do it for free. I don't do oil changes just because of mess it makes. I do to my cars, though
I do help with few hours simple jobs like what you described at no charge. But I would not even try to help friend with clutch job. Because I don't want to charge a friend money. I think I _might_ consider some job just because of sympathy and knowing how much it will cost at dealership. But I will ask for some fancy tool into my collection or whatever

AND, I will absolutely ask to clean up garage after that.
With co-workers I work on $50hr deal and only if I find job interesting. Like somebody mentioned above I try to take only newer interesting to me cars. Audi and BMW is what I like to work on. I tell people ahead that they can watch what I do. That I do it for fun. And that I'm not being cheapest around.
I also try to make more then $50hr, but this is rate you should determine. Keep in mind that you will need to redo job sometime... Like I did CV joint(split boot) and it happened that other side was done before by dealer. Dealer re-sealed bad joint so car was making this pop noise even after replacement. Try to explain something like this to regular driver? I had to replace other one for free(they bought parts of course) to prove.