The End Result
Some interesting points that we noticed before and after the upgrade (at least on the HTC TyTN), was that with Windows Mobile 5 we had a total of close to 55MB of storage space with some 49MB available. We expected this to drop as Windows Mobile 6 takes up more space, and we weren't wrong. After the upgrade we had just over 47MB of storage space with just under 37MB of free space.
Windows Mobile 5 to the left and Windows Mobile 6 to the right
However, what we didn't expect was the increase in program memory or RAM. With WM5 the total memory available was close to 49MB, and just over 25MB available when only the screen capture application was running. WM6 had about the same total amount of memory available, but had over 28MB free to use. This was a surprise indeed, although it shouldn't have any real impact on performance, apart from the fact that it would allow you to run another application or two before the device would come to a crawl.
This is of course dependant on what applications you're loading up, and having Windows Live on the home screen eats up a fair amount of memory as well, but this is easy enough to disable. Our TyTN device felt just as responsive as it did with WM5 and we're quite happy with the upgrade so far. Just remember to sync up your phone with your computer once you've installed the new version so you get your precious data back onto it.
I hope this has made your upgrade process more painless and streamlined, as you can see it's really not too hard to accomplish at all. Good luck and enjoy your free upgrade folks!