Hi Tim,
Oddly enough, I had Gary Campbell set me up a ATC machine with six tools. I wish I would have made this leap long ago.
I think my favorite part of having the ATC is not going through the Z-zeroing routine every time there's a bit change. Even if I had to go to the computer at the end of each tool's cut file to manually select the next tool (which of course I don't), I just don't miss zeroing the Z dozens of times per day. Obviously, it's so nice to have all of my tools all loaded up in their tool holders already zeroed to the table and ready to go. Toolpathing for the ATC is a breeze too. I just line up my toolpaths in the order I want them to run, load the cut file with all of the toolpaths and hit go. I go do other things in the shop until the ATC has everything done... unload/reload and go again.
I know you're aware of the time savings that comes with no longer manually changing bits and zeroing the Z, but I'm amazed in my short time with my ATC how much more productive every single day has been as a whole. It's so much easier to keep the machine buzzing than I ever thought it would be. I can't image anybody would go back to manually changing tools after experiencing an ATC.
I used to think that a guy would really have to justify spending the money to have an ATC. Now, I think you'd really need to justify not having the ATC.
Good luck!
Eric Erickson
Shopbot BT48 Buddy