Having a boss who is extremely open to new ideas, and the possible successes they bring helped. Licensing and cost comparisons had a factor. I think the single biggest decision, though, was that we wouldn't be locked into a single vendor for the bulk of our infrastructure. MS, on the other hand, no longer sells Windows NT 4.0 or SQL Server 7.0 licenses. We're using Microsoft Office 2000 Professional for our application suite.
Training is readily available from any number of vendors in our metropolitan area.
It's not perfect, and there have recently been forks in the development effort as people feel that certain functionality is more important than others. But it offers an incredibly flexible solution to many problems. I originally complained of my inability to change my SMB password from a Windows 98 client. I had installed the RPM of Samba that was included with Red Hat Linux 6.2. My other complaint was that Samba had no capacity to expire passwords.