Not to disagree with everyone else in this thread, but I've been using wyse thin clients for my POS for years. Mind you, the software involved is not RMS, but I've made no major changes in my custom POS to facilitate it's use on thin clients. What I get out of the deal is a lower initial cost for hardware, a single point for both upgrade and failure, and generic/disposable client machines. As for heightened reliability due to the absence of client harddrives, I'm afraid that is a myth. The clients which I have used have about the same failure rate as my Dell workstations, except that when they fail, it almost always requires sending the client back to wyse for replacement. Fortunately, the cost savings on the front end, allows me to leave a spare client or two at the site for quick trade-outs.
Thin clients are good for distributing applications around a lan at low cost, but they are not without shortcomings. And most of those shortcomings center around the needs of POS Software. For example...
Ports and Local Printers
The wyse terminals I use have a beautiful array of ports on their backside, but only LPT1 and COM1 are available from the Terminal Services session. The others can be used for mice or touchscreens, or are totally useless. POS software often requires ports for cash-drawers, pole-displays, mag-swipes and even usb cameras. Fortunately, most of those components can stack up (Some mag-swipes and barcode guns can ride on the keyboard. Some cash-drawers and pole-displays can ride on the receipt printer). It can be done, but you gotta plan for thin client port starvation before you start buying your peripherals.
Another issue that must be overcome is giving the POS software access to the receipt printer. Whenever you are using terminal services to run an application, the code is actually running on the server. Whenever software references memory or ports, those resources are the ones physically located in the server. It is up to you, to map the client's remote resources to that client's unique terminal session. Depending on the model of client you choose, that might be a difficult challenge.
As for the user's perception of speed, it is relative to the power of the servers. I use 2.4+ Ghz P4's with 1G Ram, SATA drives and Win2K/Win2K3 and get excellent performance for 5 - 10 clients running my POS.

Hope some of this helps!
Peace,
Colt
If it's stupid but it works, it isn't stupid