I have upgraded my "new" macbook from Tiger to Leopard and it turns out have been a really good idea. When using Tiger, I had this concept of OS X being really clunky. I chalked that up to being new to the OS and not real failings in the operating system.
The most noticeable improvement for me is in the Finder's ability to browse remote Windows network drives. Leopard's "Places" makes network drives much more accessible. The WiFi menu is not so annoying as in Tiger where it would wait and do a rescan of available networks before it could come up.
A lot of functionality under Leopard just seemed more usable. I am a bit surprised by this. I regularly switch between Windows XP, Vista and Windows 7 and while there are differences, the basics just do not seem that different. I'm sure that part of this is that I've been using Windows for over 20 years so a lot of it is just second nature to me. I guess I'm not the idea switcher as I've got such depth of knowledge in Windows that using OS X is often frustrating to me. Luckily Leopard seems less frustrating. Looking forward to Snow Leopard!