I am still using 6.2. I am waiting for the issues and questions on the DUC to clear up a bit.

I just upgraded to 6.7
I must say there are a lot of improvements on the midi capabilities of Pro Tools, along with some nice little new features that make your life easyer. (Nice new New Track functions and color coding of the tracks and regions)
All these impovements are more than welcome. Read the information provided with the upgrade, before you instal.
On the other hand, I must complain about all the shortcuts that changed. I'm not going to list them here, it would be pointless.
Once again, you will have to learn them and they really get more and more complicated.
Most of them really don't make any sense. Strangely, they only seem to work with your keyboard set to US and Qwerty. Some shortcut DO NOT work in other keyboard configurations.

If you are not using Pro Tools as your main Midi sequencer, stick to 6.4
If you do use midi extensively in PT, you should upgrade, but allow yourself some time to get used to shortcuts changes...

It seems as stable as 6.4