It appears that your experience with Leopard may not be the dominant one, at least as it pertains to installation. Apparently many users, including myself, who chose the upgrade option on installation (instead of clean install or archive and install) were hit with the blue screen of death upon reboot after installation. No message given, no cue as to what was happening, just the dreaded blue screen. There are quite long threads regarding this issue from users and initially Apple was noticeably silent on the matter. Inane tech support merely ran me through a bunch of unworthy steps before telling me to archive and install, duh. (If I had wanted to utilize this option, I would have done so. The point was that I did not want to have to reinstall many of my programs, thus I chose the ugrade option). Apple finally acknowledges the problem and has the user boot into single user mode and run a series of unix commands to remove ape modifications (the theory being that the problem was not its own, but one of third party modifications). A link to one of the many threads describing this issue is here http://discussions.apple.com/thread.jspa?threadID=1195031&start=0&tstart=0
In contrast, my vista and ubuntu installations were a snap and each loaded up in under 30 minutes, whereas the leopard installation took literally hours to complete eventhough nothing fancy was being proposed with the installation, just a mere upgrade. It is noteworthy, that vista installed just fine eventhough my system had several non Microsoft 3rd party apps, devices and drives, whereas my total apple designed system was unable to manage an upgade.
Once loaded, leopard seemed incredibly slow as compared to tiger, but I suspect that the initial indexing activity service of spotlight may have something to do with this. In my opinion, the most impressive feature is the time machine backup utility which finally allows me to retire retrospect. The other features, such as the stacks and coverflow features, are frankly underwhelming in my opinion. On first look coverflow seems more suited to music and itunes than to other types of files (probably has something to do with the lack of cover art) and stacks while it can be useful, is not earth shattering. With the caveat that I have not played with it much, the spaces feature seems similar to what has been available in linux gui for years and is appreciated, yet not revolutionary.
Although I would have bought leopard under most circumstances (as I am a techno junkie and must have the latest os, gadget, software, etc. for each platform), leopard does not appear worth an "upgrade" from tiger. Tiger is a decent operating system as operating systems go and leopard does not add much to it aside from what appears on first glance to be cosmetic improvements.