We started a nanny share in January, and so far, it's going really well. We get two extra days of babysitting/week for the same cost overall, Jarmila gets paid twice as much (for twice as much work, granted, but she wanted the money and the work), and Kavi absolutely adores little Gavin (currently three months old). I think she thinks he's the best dolly ever. She gives him his binky (or tries to), tries to give him his bottle, tries to help Jarmila change him, etc. and so on. Kavi cries when Gavin leaves at 3:30 each day, and we have to reassure her that yes, he's coming back tomorrow.
Verdict: nannyshare highly recommended, especially if you get along well with the other family. Tim and Angie are great, and Gavin is a sweet baby.