I have a pair of pro900 which I find to be excellent for reference monitoring. I bought them to produce on the go while on tour this summer. The freq reponse is a bit bulgy on the bottom but I got used to it, and the top is bit harsh at times but still manageable. Overall I would say they exhibit remarkable clarity in all bands.
When I listen to pro900 mixes on Mackie HR824s I am often surprised by the absence of a lot of typical earphone-mix issues, especially in the low end. Imho pro900 are a better buy than equivalently priced monitors.
My previous monitoring cans where Sennheiser's HD280 64 Ohm. So a big jump in quality of course.
However from previous studio work I know Beyerdynamics DT770 and Sony 7509 and personally I prefer the Ultrasones pro900s, also because the design, albeit closed, is optimised for long hours of listening.
That being said, I have been thinking to buy my girlfriend a pair of 2500 but I keep reading terrible reviews everywhere on the net, so I presume that if you want to get the right value for money from Ultrasone perhaps you need to jump to the very expensive models.