Colin, the Brightscreen came a couple of weeks ago and I've had the chance to use it now under a fairly wide range of circumastances. To be honest it's a specialised purchase and I wouldn't recommend it unreservedly for all users.
On the positive side it's about 1/2 stop brighter compared to the Contax screen (when measured with a remote light-meter head, taking a reading directly from the screen itself), the huge microprism collar is a real bonus for off-centre focusing, and the diagonal rangefinder suits me very well when flipping between vertical and horizontal for portraiture. Most importantly it provides a crisp image with plenty of focusing "tooth".
The major downside is that normally you would dial in an exposure compensation for a brighter screen and then just forget about it. Within the Contax 645's optical chain it's not so simple. Spot metering measurements are taken before the focusing screen within the camera body, but centre-weighted measurements are taken from the screen itself within the AE Prism. What that means is you need to deduct 1/2 stop from a centre-weighted reading, but a spot reading needs no adjustment. Not particularly convenient!
It all comes down to your personal preferences, I guess I'm a little obsessive about focusing screens, and regard the viewfinder experience on the Contax 645 as a weak feature of a generally outstanding system. As a spectacle wearer I'm unable to see all four corners, magnification on the AE prism isn't great, the standard screen, IMO, is a country mile behind the Hasselblad Acu Matte, the limitations of the auto-focus system means I'm often manually focusing while the standard screen is plainly designed for auto-focus, and my own personal style leans towards wider apertures with faster shutter speeds (which is why I want a Zeiss lens equipped system in the first place), so I place a real premium on focusing accuracy.
For my purposes the Brightscreen makes sense, I'll put up with the metering inconvenience for even a marginal viewfinder improvement. However, it's a trade-off that may well be innapropriate for many users, especially if they have younger eyes!