I few weeks ago, I installed MovableType 5 here to play with, I had toyed with MT4 a while back, but there were too many template differences from MT3 for me to make the switch at the time, and I had no good reason to upgrade. The increase in comment spam of late, though, finally prompted me to make the upgrade, as MT5 had improved tools in this area.
After some slow going at first, I finally managed to create templates that matched my MT3 blog, and from where began working on converting my style sheets. This process went a bit quicker, but still took a bit of time to work through.
One of the main issues that I struggled with was the lack of adequate documentation with regards to building a Theme that could be installed, and how to create different Styles for a given theme. It took some time piecing bits together from various sources, but I've finally managed to create a working theme with multiple styles. I'll be working on a write up of this that should hopefully help others to get through this process a tad faster than I did.