We also had two large (U8) Compaq ProLiant servers that we used to run Oracle DB. We didn't have any problems with them, and only replaced them with newer faster Dell servers later. The redundant PSUs and hot-swap drives were the main reason we ran our database on them (one master, the other a standby slave). Those server were later used for our beta site (used for staging/development).
We later ported the backend Java service to C++, which most likely would have run fine on the UltraSparcs, but I think we had already sold them or returned them back to the vendor. I really wished Sun's UltraSparc T1 & T2 CPUs had taken off. I think they could have really good CPUs, but it was hard to compete with x86 because of all the software that was already compiled/optimized for the x86 architecture.
For us it was important to be able to fix problems quickly. Having to call in a service tech. to fix a hardware problem just increased downtime. Also we just had really bad luck with the IBM Netfinities, all 4 servers had a bad motherboard which failed in each server at different times. After that we didn't have any problems with them.
Personally I prefer to deal with generic hardware that I can service myself when a problem happens. But for some companies, it is better to have managed hardware with service contracts.