I had heard that Tiger (10.4) is faster than Panther (10.3). I believe it is. It also seems to run hotter on my PowerBook 15" as the fan runs a lot more than it did with Panther. That shouldn't be a problem as I use a Podium Pad to raise the base of the machine off my desk.
Spotlight works as advertized but it doesn't seem to be able to access Quicken files, which is a pity. It is incredibly fast and can sort the results in a variety of ways.
Widgets are interesting but. . . so what! I will probably use a few of them. One, Rabbit Radio, looks interesting but it doesn't keep connected. Others (like Rabbit Radio) are USA oriented. Weather for London, as an example, turns out to be London West Virginia (which seems to have a better summer!). The flight tracker has its positioning option disabled (no doubt for security reasons).
Others have commented on Mail's new interface and I agree - it sucks. But there are some other new features that are useful.
RSS feeds are now available on Safari but I shall keep using NetNewsWire as it has a better interface.
Sync works very differently - a total rethink is needed by the user who is familiar with the old Sync application. I have not tried to sync my phone yet but again I have a third party tool that works fine.