It'sIf you got here because of the question's title (how do I use the Pi as a thin client) rather than the question's content (how does it perform as a thin client), it's probably worth having a look at the RPITC (Raspberry Pi Thin Client) project too:
It comes with a choice of clients (VMWare View, RDP, Citrix, X2Go, ...). I've only used it for RDP so far (RDesktop and XFreeRDP), performance there is more than satisfactory.