I have the drivers working absolutely fine on my Win7x64 machine. So far it even seems to be more reliable than XP too. But then I haven't used it so much yet. No lock-ups on exit either at the moment.
Given that that's not much help, here's a bunch of chat which is mostly irrelevant too....
To combat Midex8 unreliability in the past under XP, I have established a method of working where I steadfastly clear out unnecessary midi data. For example, I always erase Aftertouch unless I'm actually using it. I also thin out aftertouch and pitch-bend data by playing it in and then redrawing it using 'straight lines' using the orig data as a template, which causes the data to be drawn at 'quantize' intervals. Sometimes I use automation instead because it has straight lines I can edit much faster.
So, in effect I keep the amount of midi data down, and I have found this helps to avoid midi lock-up problems. Problems like the device going offline or simply seeming not work anymore, or notes being dropped.
I still have problems chasing bank and program changes though. And I still have a problem with Steinberg's choice of which controller numbers to chase and reset - they're not my choice! But that's Cubase not the midi device.
My Midex8 is plugged into an 8 port usb-powered hub which itself is on the end of a 5m cable usb cable. I have many other things plugged in everywhere, sometimes via extra hubs, including my Fantom, my Triton Extreme, another usb keyboard, wireless mouse and keyboard, hard disk docks, hard disks, 3xiLoks, Cb dongle, card reader, m-audio sound box etc. The only problem that I have is sometimes the 8 port hub needs unplugging and replugging to bring back the Midex8. Also sometimes my mouse and keyboard stops working and I have to plug it in somewhere else. You get used to these things, but mostly it all works smoothly, usually caused by plugging another USB device in somewhere.
And so far I have not had any lock-ups on exit on Win7x64+Cb7, but again, I've not yet really used it for the large projects which I've used on XP+Cb5 - where it did lock up for a long time most of the time. The next few months will be a test for the system as I'm working up my new projects that have been started on Cb7.