I don't expect everything to work perfectly on launch. Ubiquity has issues, but I am still a strong advocate. However, since nobody else on the web seems to have anything bad to say about mashed life, I will fill in the niche.
I found 3 issues with the service.
1) It didn't work for all sites.
The one button that is supposed to give you access to sites is a bookmark (or more precisely a bookmarklet). That bookmarklet scans the page you are trying to login to and pastes your username and password into the appropriate fields when it finds them. Or, more precisely, if it finds them. It didn't work for 2 of the 10 sites I tried before gave up on the service. One of them used frames, the other had an unusual form. Mashed life was useless on both.
The Firefox password manager can assign an unlimited number of username and password pairs to a given site. Mashed life will only store one. I have more than one Google account. I have heard of people having multiple Facebook accounts. The second account will have to be committed to memory, or stored in some other service. but Mashed life is not the final solution for this situation.
Kevin pointed out you can use multiple accounts for a single service. Mashed life will then direct you to "a selection page to ask you select which [...]account you want to log in to".
3) Can't distinguish multiple login options.
My web host has a login form that has two buttons. One is for site maintenance, the other is for it's affiliate program. Mashed life's one button login automatically logs me into the affiliate program. I would have to use another service to access the maintenance tools.