I don't know about anyone else, but here's what we have...
We've got a WinNT Deployment Server (soon to be W2K), 2 Citrix servers (load balanced), an AS/400 enterprise server, and two or three fat clients.
Most of our users are using PC's or thin clients and are accessing OneWorld (as well as other apps such as Lotus Notes, Word, Excel, etc) via the Citrix clients. I've got a fat client (being the sys admin). We also have a "community" fat client that our users use for report development, and our consultants use for app development. We will be installing a fat client for two of our "critical" users (one in customer service and one in accounting) for when we have problems with Citrix (rarely, but we do run into some hiccups at times.)
I'd have to say that we've got about 20-25 users using the Citrix boxes altogether right now, and will be adding more users next year, as we bring up more OneWorld modules (maintenance crib inventory and work orders are currently done in World) next year.
I hope this little bit of info helps.