I've used the MTX trunk bag/rack setup for almost a year now in my commute and really have only good things to say about it. I'm no speed demon while I'm riding so shaving seconds off at the beginning and ending of my commute is very satisfying.
The bag's well made, VERY quick on-and-off, and reasonably rain-resistant...except for the time there was a downpour the entire commute. Like someone else, I'm looking forward to ordering the rain cover...