There's been lots of reviews about Leopard so I'll try to cover some aspects from a practical point of view.
First is stability; I decided to do a clean install; I've upgraded a couple of times before for Panther and Tiger (without problems) but have also migrated from Power PC to Intel so I felt a clean install would blow any potential glitches away. I'm glad I did as Leopard runs far more smoothly than Tiger ever did. I reckon it's about 25% quicker on average. I know it depends upon what you're running and your spec etc etc, but to give a simple example, my Macbook used to start up in about 20-25 seconds on Tiger; it took just 15sec the first time I restarted with Leopard.
Cover flow is not a useful as I thought it would be. Yes it looks fancy but I prefer to have overall visibility of my files within a folder and whilst it might look fine on a 24" iMac on my 15" Macbook I just find cover flow view a little too restricted to see quickly what files are present. Bottom line is I hardly use cover flow. Quick look on the other hand is excellent and I only open files when I want to work on them. Quick look is quick, easy and when combined with Spotlight is a potent search tool. I really notice the difference when using Windows at work.
Time machine is really really good but I'm not sure why it doesn't work with iTunes. To give you an idea of storage, I back up everything from my Macbook (about 60GB worth of files) and after a week Time machine has taken up nearly 80GB and it's going up and up. For 80GB of files I'd suggest a 320GB HD (a factor of 4) is a good rule of thumb if you're thinking of buying an external HD for time machine. That should give you at least a couple of months worth of back dating with room for comfort. Be advised though that Time machine doesn't work with drives connected wirelessly even through Airport extreme - a real shame, especially for laptops but one I'd expect to see remedied soon as Leopard was originally advertised to have this capability.
Spaces takes some getting used to. I tend to close or hide apps when finished with them and spaces can get a little confusing when working in multiple apps in different spaces. I'm using it more and more, and I'm sure with time it will be incorporated in to normal working routine. Stacks are good, but a slight shame they've removed the ability to right click a folder in the dock and access sub-folders.
I really like Safari 3 as it's much improved from the beta version. Having used it for a week it's really really fast, displays pages perfectly, and the web clip widget maker is great, although I don't see how to save a widget you've made yourself. A little frustrating as once you've closed it, you need to remake it. Maybe I'm missing something?
Front Row is a definite improvement, it's faster, slicker and miles more stable. The animation to access front row has gone, but I think this is a bonus - with dashboard and time machine it would be too many screens dropping away and being superimposed. I have a slight issue when connecting my laptop my my HD TV in that front row appear but doesn't pay video but it plays the sound. Might be specific to me but still a concern.
iChat is still limited. It works with google talk, AOL and dot Mac accounts - Apple in my opinion really need to expand this out for yahoo and msn etc for make it useful. I have a dot Mac account but I don't know anyone else who does so I can't chat with them. A shame really as it's a waste of a good app.
Help for spotlight is a really cool function. Just be advised that if you don't know the name of the command then it won't appear in your search. A little obvious really but typing 'justify' rather than 'align' might not yield the result you're looking for. But now I'm really nitpicking! The 3-rd party apps I've installed work well, a few (but only a few) are still incompatible but these are getting updated every week; and iLife 08 just purrs. The criticisms I've made are all minor and don't detract from the value of this well-built and thought through OS. It's well worth the upgrade, fast, slicker and more co-ordinated.
Don't get bogged down with the US/UK price comparison argument. Now one vetoes Levis jeans because they're cheaper in the USA. I'm happy with the price for the OS - it's a fair price for the UK, it's great value for money and is far far cheaper than windows. Definitely a good purchase.