Your best move is to read the DiRT help file and view the training videos for it here:
http://www.ciscounitytools.com/App_DisasterRecoveryTools.htm
that way you don't have to rely on 2nd hand anecdotal information!
The 200 user thing is a comment I made in the help file that DiRT is not an appropriate tool as a free replacement for a regular backup for large systems. It's designed as a migration tool first and foremost such that you can move around between hardward, configurations, back ends, domains etc... without losing information. You can use DiRT on any sized system regardless of the number of users. If you select to backup messages just be aware this uses ExMerge (which I go into detail on in the help file) - this will work with any number of mailboxes but it's not the most efficient message backup mechanism for large numbers of mailboxes. For a one time move it's OK as long as you have enough HD space to handle the backup but as a regular backup mechansim that runs nightly/weekly it's not a good idea to use that option to avoid having to purchase a full Exchange aware backup mechansim.
I put this comment in the help file specifically because some folks were using DiRT on very large systems (thousands of users) and backing up messgaes every night - and discovering that it's not the best solution for that.
If you don't back up messages you can go wild. If you're just doing a one time migration you can go wild. But if you're trying to save a few bucks for a customer and offering DiRT as the end-all-be-all of regular backup tools that avoids having to purchase a fully functional Exchange aware backup tool and they have more than 200 users, I'd seriously reconsider that approach.
