nope, i havent but from what i hear, lion only has superficial changes like more "app-like" interface and scrolling, and thats about it.
and im not sure bout this but i heard you can only upgrade to lion from snow leopard, which is crap because i'm still running on leopard and if i wanna change to lion i would need to upgrade it twice. i dont think it was worth paying twice the upgrade for such minimal change.
Actually - the big changes are mostly in new features rather than in new look. At first glance, you won't see much change.
The big things for me:
1) built-in full disk encryption that's really easy and very well thought out, as well as support for encrypting external drives too
2) System-wide auto-correct that's very nicely implemented, when it makes a change it underlines it in blue to show it's done something, and it remembers what you had typed as long as that blue is there, so you can revert if it got it wrong (which is doesn't do often unless you REALLY fat-finger it badly)
3) system-wide auto-save and automatic do
ent versioning (not supported in all apps just yet, but more every day) - no need to hit save, just like on the iPhone you just do stuff and it's safe. As soon as you pause in typing or switch to another app your stuff is saved. Your previous save points are retained for 2 weeks (by default), so you can show the current and all previous versions side-by-side, and even copy and paste between them - this is fantastic for all those times you accidentally destroy an important file (I've been known to do that a lot)
4) Apps save state when they're closed like on the iPhone (not all apps yet, but more and more of them each day) - everything is remembered, which files you had open (even ones you never saved), where all your inspectors were, even where the cursor was - and this happens at an OS-wide level, so if you reboot, all your windows in all the apps that have support will be right back where you left them even after a reboot
5) lots and lots of multi-touch gestures added to help you navigate all your windows and spaces etc.
As for cosmetic stuff, here's some of the highlights there:
1) Exposee, Spaces, and Dashboard have been merged into a single interface called Mission Control, it comes with a button on the Dock, but that's not how you want to use it, you want to tie it to a hot corner or to a multi-touch gesture or mouse button (this works well with gaming mice, old Mighty Mice from Apple, the new Magic Mouse from Apple, and the Magic TrackPad, as well as all laptop trackpads)
2) Full screen apps - when you make an app full screen it becomes a desktop of it;s own, so it's as if a special space was made for it in the old OS X Spaces. You can use gestures or key combos (ctrl+left or right) to navigate between the full-screen app and your other apps or the Dashboard. This is true full screen where you see nothing but that app like on the iPhone, and it's amazing for getting things done. When you don't want distractions you just full-screen the app your working in and you're free to get on with shit
3) An optional new app launcher that is basically like an iPhone home screen for your Mac - all your app icons are there, and you can re-arrange them into multiple screens and into folders. This is great for people who are used to having a Windows desktop full of icons, or for people who thing the dock is too small for all their stuff, and who don't use SpotLight or third party app launchers to launch apps (I never use it, but it is nice, and you can bring it up with a gesture or a custom mouse button or a hot corner)
4) the scrolling change - I just ADORE natural scrolling and the removal of scroll bars, but some people won't want to change so you can change both settings back to the old way if you like
5) Some little UI tweaks - no more childish rounded buttons and progress bars, instead things have gone a little more mature and square looking - most people won't care about this, but I like it a lot. There are also loads more animations when things appear and and disappear - some people won't like them, but they are very quick so they don't waste time.
That's just the really big stuff, there are also lots of totally re-designed apps, like Mail and iCal, and of course for people coming form Leopard the App Store will be new.
As for how you upgrade, right now this second, you need to upgrade through 10.6 to get to 10.7 from 10.5, but that's going to change within the next few weeks. Apple have announced they'll be selling a full version of Lion on a USB thumb Drive for $69 "next month (they said in mid-July)", so if you wait for that you get to go straight from Leopard to Lion. I upgraded my parent's machine as a double-upgrade and it worked flawlessly and in less than 2 hours.
I've been running Lion for a few weeks now, so if you have any specific questions please ask.
BTW - as a Leopard user you're goning to need to upgrade to at least 10.6 in the next few weeks, Apple only support 2 OSes at a time, so Leopard is now end of life as far as security updates etc are concerned.
Laters,
B.