We've improved the 'Reply' behaviour so that it will use the X-Delivered-to address in preference to falling back to the default personality. Hopefully the only difference that you notice is that you have to correct the From address less often!
(Previously if there was no address in the To or Cc which matched a personality, then the default personality would be used when replying. This could be annoying when replying to a message of which you were a Bcc recipient, or a message that was addressed to a throwaway address in your own domain.)
This also means that you may not need to use as many personalities, but that depends on how you use them.