I had the Pro 900s for a time and wasn't very impressed with the isolation. During quiet parts of a song, I could quite easily hear extermal noises, say a TV at 10 feet which was at a reasonable volume.
I also think they were quite poor at preventing leakage. I never felt comfortable using them in my office with folks sitting within a few feet of me. In my opinion, when they were used to listen to music at an enjoyable volume, which for me is definitely NOT at a damaging level, they'd be almost as leaky, and thus annoying, as ear buds.
In the end I sold my Pro 900s, tried a few different closed earphones, found they were all much the same, so went with a really good pair of IEMs instead (the ACS T1s). The IEMs have fantastic isolation, zero leakage, and superb sound quality.(NOTE: I found the cheaper IEMs, say sub $100, weren't so good.)
There is an isolation chart you can look at on the 'specs' tab here:
http://www.headphone.com/headphones/ultrasone-pro-900.php. I'm not sure how accurate it is, but it seems to reflect my experiences. The Pro 900s get circa 15 dB isolation. Good IEMs give 25+ dB isolation. That may not sound like much more, but because the dB scale is logarithmic, 10dB is twice as much isolation.