I've used the A&I mailers quite a lot for E-6 and K-14 when they were doing it (including E-6 and even K-14 push processing) and occasionally for C-41 machine prints, but never for C-41 B&W, and I've been quite happy with them. The quality is just as good as the best NYC pro labs at about half the price using the mailers, but obviously with a longer turnaround time.

I recommend bundling mailers and sending them in a box. I usually use the small Priority Mail boxes that are about the size of a book or a VHS tape. Before then I sent them separately, and I've only lost one roll that disappeared around 9-11 during the anthrax scares, when postal facilities were being shut down.