Sorry for the double post, finally got my copy in the mail today (took nearly a week to get from Apple's depot to me...), so I thought I'd give a bit of a review.
I got the Family Pack because I own two Macs; I'm an honest person so I wouldn't buy the single-licence even if it is "just one" extra computer it's getting installed on. Did the upgrade on my MacBook Pro, because it'd take a long time to get all my data back from the backup, I just couldn't be bothered. Did a clean install on my iMac, though.
Amazingly, there's actually several things that you will only see if you do the clean install and not the upgrade:
- The default size of all icons in Finder are now 64 x 64. If you do the upgrade then they'll stay at whatever size they were in Leopard (so for my MBP they were still the old default of 48 x 48 ).
- The Applications folder is now also in the Dock as a stack, along with Documents and Downloads like in Leopard. If you merely upgrade then it is not added to the Dock and you must do it yourself.
For the most part, it looks almost identical to Leopard. The first major difference is that Apple has changed the default color gamma from 1.8 to 2.2, so everything looks a lot more vibrant. Secondly, the right-click Dock menus have been changed to white-on-grey as opposed to black-on-white like in Leopard. The 'Empty Trash' button is also now black as opposed to grey. The date can now be shown in the menu bar; that took Apple a long time to add for some reason. Animations are a lot smoother, if you draw a selection box on the desktop and then release the mouse, the box will quickly fade away as opposed to immediately disappearing like it used to. Applications can now be minimised into their app icons to save space on the Dock. Exposé has been majorly redesigned, and I'm still trying to get used to the massive blue highlights that now surround the window you select. Very small changes in the end, but it makes the experience a lot more refined.
Now, speed. Both of my systems, regardless of whether it was an upgrade or clean install, are very fast now. All the icons in my menu bar load the moment I log in and everything is up and running right away - applications open for the first time after just one 'bounce' on the dock, and I'm yet to have the system choke on me at all (it used to do it quite a bit in iTunes under Leopard). The only thing I noticed is that it takes slightly longer (ie: a few seconds) to boot up, but that honestly doesn't bother me. If it was maybe 30 seconds or more then I may complain, but the difference is negligible here.
Quicktime X has been very interesting to use, mainly because it's actually become so simply - though I really do love the new interface. I ended up getting it to install Quicktime 7 just in case I ever need its features. I also installed Rosetta because I'm sure a couple of my apps are PPC-only. I'm not too sure what to think of the system counting in Base 10 now, either, because some of my apps (like iStat Pro) still count in Base 2; they clearly need to be updated. It's also going to cause some conflicts in regards to using it with Windows or swapping external drives between different systems, but whatever, at least it'll stop the whiners that think their drives are the wrong size

The install process is also very quick; it took just 20 minutes to do the clean install on my iMac, from putting the disc in to when my account logged in for the first time.
The biggest thing I noticed is the dramatic improvements in battery life that I am now experiencing. I have the very last 15" MacBook Pro to have the removable battery (it's the early 2009 model), which Apple advertised as getting 5 hours at the most off a single charge. With Leopard, being on the internet while using the battery I would be lucky to get 2 and a half hours out of it. With Snow Leopard, I've been using it for about an hour and a half and still have 70% remaining, with it telling me I still have about two and a half hours remaining. Gaining a full hour and a half from an OS upgrade is, in my opinion, amazing.
Overall, I am very impressed and recommend it for anyone running Leopard. The upgrade process is painless, and you're left with a system that is much more refined and bug-free. Hopefully Apple continues this with 10.6.1 and beyond.