I think it's definitely a viable business - it's available in every city in the US now with a ton of bikes. A lot cheaper than any competitors as well.Interesting idea.
No way I could see myself renting out any of my personal bikes, (i think it's geared more for people with small rental fleets anyway) but I see a lot on there that look like nice bikes for relatively cheap day rentals, I could see this as a good way to get some riding in when I'm on a trip someplace without my bike. The only question might be the condition of the bike?
I'd love to hear from someone who has tried it as well.