I just did it yesterday via the App store and it's awesome! Took under 30 minutes to complete the entire process. Changing and entire OS on the Mac is easier than trying to install a program on a PC!!
I use MS Outlook and Firefox so don't really know the full extent of the upgrade for the two apps but everything else is great.
Here's a list of most of the upgrades from Apple:
http://www.apple.com/macosx/whats-new/features.html
The only thing that was annoying was the fact that all the apps that came with your Mac (i.e. Safari, PhotoBooth, QuickTime, etc...) duplicated itself. You have to go delete the old version one by one. Also, I noticed that there are some crashes from time to time wtih certain apps. Though they are working hard to upgrade everything and there are constantly new updates for your Mac.
Mission Control and Launch Pad is awesome. I now have multiple desktops that I can switch between with ease. Have a desktop pretty much dedicated to 141 forum! lol...
The coolest thing I found (not sure if they had it in the previous OS) was the text to speech function. For someone like me who don't read Chinese but can understand and speak, I can simply highlight the characters in Chinese and have the computer read it out to me!
