Register ClickyStill not really good enough. From experience a T2 (coolthread) server will be jogging compared to the running UltraSPARC and the sprinting x64 boxes. For an application server, it does come in handy with a high load. Where the UltraSPARC or x64 servers will get crippled by high workloads, the Coolthreads will just keep on jogging.
They make good web servers by all accounts, but as I said, not a wonderful app server.
The updated LDom code is very good news, they aren't difficult to use currently, but they can only be made easier!