Macbook Pro 2.33ghz 3gb RAM.
Upgraded from Tiger absolutely no problems. Took about an hour.
First impressions:
-- Very nice. I love the UI. The 3D dock is amazing and even reflects live movies as well as reflects the icons placed onto the dock itself. I'm in love with the Aurora wallpaper.
-- I write a lot of code so the first thing I did was open Xcode Tools and it crashed. I logged out, restarted. Nothing worked. It kept crashing on launch. Popped in the Leopard install disk again and overwrote the Xcode install and it worked fine after that.
-- Time Machine is awesome. I connected by 250gig HD and Time Machine asked if I want to use it as backup and I said yes. That's about it. It does take some time to backup though. A complete backup of my system took about 40 minutes with the initial backup being 58gb. Initial backup of your system takes exactly the amount of space that is currently in use on your internal HDDs. Time Machine does not recognize NTFS partitions as it completely ignored my Windows NTFS partition.
-- I love spaces. I am a chat freak and chat with my fiance 25 hours a day. Now I can set her personal full screen chat window in a separate space and away from all the clutter I am working on at the moment. It's awesome.
-- Quicklook is AWESOME! Seriously Quicklook ROCKS! I watched all my little videos again just to watch them in Quicklook preview lol. But seriously, it previews almost everything. Very very cool.
-- Booted into Windows and put in my Leopard install disk. Auto-Install Wizard updated all my Windows drivers though I haven't tested anything there yet. Still playing in OS X. However, the installer crashed on one driver. It was something like "USB Composite removal driver" or something. I got some exceptional error and Windows went on to install the rest of the drivers after skipping the one that gave the error.
-- Some of my programs aren't compatible with Leopard of course. Some noteworthy ones are: Parallels (Build 4560), LiteIcon, Cocktail, Tinkertool, Onyx and Shapeshifter. I upgraded to Parallels build 5160 and it worked then, however I seem to get an error when Parallels tries to mount my Windows Volume into OS X. It give some timeout exception although Windows itself works perfectly on Parallels.
-- The rest of the Applications are running smoothly as they did in Tiger. I have noticed a considerable speedup in the launch of all applications and especially the ones that come packaged with leopard. All of those launch on one bounce or two.
Overall I am extremely happy and impressed with Leopard. It's made my macbook pro so much better.
There are two things that have me kind of disappointed though. Firstly, Perguardian is not compatible

Yes, I download torrents. There are legal torrents out there you know. I know I have nothing to worry about if they are legal but Peerguardian just gave me peace of mind that my machine is not being intruded. I usually left it on even if I wasn't downloading anything. It was an awesome program and I REALLY hope they update it to work with Leopard.
Secondly, You can't change your Internal drive Icons anymore! I tried. If I'm missing something someone please explain how. But When I click on my Macintosh HD icon, the "Paste" button doesn't light up. I've confirmed that the Icon I want to use is on the Clipboard and it still doesn't light up. I am forced to conclude that you can no longer change the internal drive icons. WHY APPLE! WHY!