I've always used exim without giving it any thought and its never given me any problems - although mine is not a large site. Exim is solid and fast enough and works nicely with maildirs. It runs nicely alongside Courier-IMAP which I use for the message store and is also a native of maildirs land.
Rather than worry about MTAs we should really concentrate on finding a fix to the problem of message threads in the notifications. It would be really nice if we could crack that - I know its been discussed before and there is no easy solution but I thought I'd mention it :o)