I'll speak about Lion, DON'T
I was having some issues with my machine, so rather than do a complete re-install of Snow Leopard, I upgraded to Lion. It has been a nightmare.
I can't even restart my computer without having to boot into Recovery Mode to run the Disk Utility. I've had horrible memory management issues (meaning it gets really slow). I don't feel like the issues with my system were improved at all.
I have read that a clean install, ie, wipe the hard drive and start over eliminates these problems. Still, there are other things that make this a horrible operating system. Lion eliminates Rosetta, so if you have any older applications, those will no longer work. The toolbars have very poor contrast, it's hard to see what is a button and what is disabled. The scrollbars on the side of many applications auto-hide, meaning you have no idea at times how far down a page or document you are. Important folders, like the Library, are entirely hidden from the user (makes things like adding or removing fonts very hard).
In short, I am dreading having to support anyone running this system as it works right now. I am not a fan at all and will likely be formatting my hard drive and starting over with Snow Leopard this weekend.
UPDATE
I finally found another show stopping but in this pile of dreck known as Lion. The CIFS share from my NexentaStor system at home doesn't mount properly through Finder. I can authenticate and mount the share, but all the permissions are screwed up. I am shown the folder, but told through Finder that I don't have any permissions to view the content.
Now, you wanna know how retarded this system is? If I put it to sleep while connected to the drive, it suddenly allows me to open it and see the contents. But don't dare try to open a folder inside the share, I don't have permission to those apparently. Oh sure, I can get to those by opening the folder from the command line, I can even do everything I need to from there, it's just the Finder that's broken. GRRRRRR!
