Do you mean portable as in on-the-go, or transportable as in taking it to hotels and using them?
Rather poor for either. The TH-900 leaks sound, has an unwieldy and long cable, and is a bit heavy for portable use.
As for transportable, the box you would use to transport the TH-900 in is very large, not easily stowed away in luggage.
Not to mention the exquisite finish on these things and the heartache you'd feel if you ever scratched the cups. I'd shed tears.
I agree totally with the above, but have to add that if you do want to use them as part of a portable rig (against good advice :) ), they sound great out of my iPhone 5. Plenty of volume and dynamics.