Bumped up the X5 (first - as it's under warranty) then M3 this weekend to V29.1
All went well with the upgrade, although I find the momentarily black screen off-putting at the start.
Taking the M3 to Goodwood's Festival of Speed yesterday I can vouch for improvements already seen while using the car on roads I'm familiar with over 250 miles:
1) Routing algorithms are better. The previous version - 27 - used to suggest really nonsensical routes, especially with the tendency to use motorways where 'A' roads would be better.
2) Speed of map rendering is improved.
3) Re-routing seems to take longer but the results are more intelligent when they arrive. Worth the trade-off.
Will post any more stuff I see after using the X5 tomorrow for a well-trodden journey.

(Incidentally - used Alcohol 120% latest version; burnt at "Maximum" speed with Burn Proof Pioneer DVD Writer on Verbatim 52X CD-R)