Hmm, from what I have read Apple have spent a lot of time sorting out various security bits and bobs within this update. Some Microsoft peeps have argued that its really a service pack update and should have been free - perhaps they have a small point? There are a few differences to the UI which make it a bit more easy to use. A couple of eg's which I thought were just for show - but I seem to be using them a lot, are the new methods of viewing docs & movies without opening them up. On Leopard you can click space bar and you get a good look at the doc/movie. However now you can make the thumb nails a bit bigger and play movies/flick through pages of docs right there in the finder window - so if you know a specific layout of a file (often in my case a journal article/statute/case law report) you can find the one you want without having to mess around (or alternatively just be more organised than I can sometimes be
A not so well known fact is that many machines will not run at 64 bit unless you hold down 6 & 4 when booting up the machine. Though Apple say even if you do not, when you use a 64 bit software it will work. This is an area of behind the scenes which I think will become useful as more 64 bit software progs become available. Snow Leopard provides developers with the tools to code properly.
One of the biggest things for me is that Mail can now do MS Exchange and I have been able to fully remove all traces of Entourage (Outlook) from both my macs. The other is the new voice over changes that they have made - when my MacSpeech update arrives I expect it to be even better as it should take advantage of these new improvements.
If you use movie editing software such as final cut I have herd that the new quicktime is an issue at present. I herd a colleague tell me that there is a list of incompatible software on the web somewhere - I've not had a look to find it as I only (so far) have 2 issues and both companies have said they are working on upgrades. If you are going to do it, I'd seek that list out first and check compatibility with all of your software.
So is it worth upgrading - in all honesty its a yes and no answer. If you have compatibility issues, wait until upgrades for those problem softwares are available. If you want to feel a bit safer (I never felt vulnerable before) that your mac is secure then yes its worth the money. If your thinking of upgrading to the new iLife and iWork at somepoint perhaps worth waiting until the new box set is out.
Edit: Forgot to mention the vastly improved expose function...