I'll just add my 2 cents and agree with cobol. Though I'm a VB developer, my firm is primarily an AS/400 shop, has been from day 1, and S/36 before that. Over the years, our NT boxes go down weekly (no surprise), but I've seen our hp9000 go down, and even our rs/6000/aix fail. But we have 2 AS/400's, both used for heavy development, training and our internal business systems.
We have brand new programmers who don't know what they're doing trying all sorts of strange, bad code on them, and highly experienced cutting-edge rpg (can rpg be cutting edge?) programmers testing the limits of these machines with everything they can throw at them, and every kind of user in between. I've been at this firm 4 years and neither one has *ever* abended. Ever. Nor have they been hacked, and they're on the internet with clients dialing constantly. I'm primarily a PC developer, but I've gained a true respect for the 400.
I now have a project using VB over DB2 that's about to go to a client, and not only is the performance absolutely blistering--the most amazing thing is that when we install the system at the client, I can truly sleep at night knowing that no matter what kind of maniacal stuff the users will do, the machine will not go down. After so many NT based apps--that's a strange and wonderful feeling.
--Jim