The Boston did stirling service with the RAF, and the Ventura did quite well despite it's civilian origins. According to Martin Bowman, the Mitchell suffered some teething troubles with internal systems, but these were minor and the type went on to give good service.
Personally, I always consider the B-25, B-26 (Marauder) and A-26 (Invader) to be in a league above the A-20 and Ventura due to their size and bombload, although this is purely subjective. It is a shame the B-26 suffered from a 'widow-making' reputation, my understanding is that it was not an inherently poor machine, but did require more skill to fly than it's contemporarys.