Underwhelming
I have to say, Tiger's been a bit underwhelming for me. Before it was out, I bought into all the hype about how useful the new features were going to be. I believed it would change how I use my computer, for the better. I was excited. But it just hasn't been so.
Dashboard is nice, when I remember to use it. That's not very often. About all I do is check the weather report in the morning, but most days I forget to do even that.
I've used Spotlight a grand total of one time. It didn't find what I wanted.

Turns out the file I was looking for was already backed up to CD and no longer online. Which is exactly why I said from the first moment Spotlight details became public that it
must optionally maintain a database for offline media. You could have a preference to automatically index CDs and DVDs when you insert them (or have a little popup come up to ask if you want to). Then when you search in Spotlight, if it finds something relevant on a backup disc, it tells you the name of that disc.
Spotlight also doesn't even attempt to index network shares. I tried everything possible to get it to do this, but even when it pretended to index network drives, nothing ever came up in search results. I gave up. I realize that it can't provide realtime updates, but advanced users ought to be able to schedule weekly indexing of network drives for Spotlight searching. These two features would have made it insanely useful to me, as my data is spread out among several computers and on dozens of backup discs. The stuff on my Mac is fairly well organized, so I haven't needed it to find anything locally.
I've found that the Tiger features I appreciate most are what I'd marginally call bug fixes from Panther. Stuff that probably doesn't affect the vast majority (99.99%) of users. Like in Fast User Switching under Panther, if my screen saver was locked and I switched back into my account, I then had to enter my password a second time to unlock the screensaver. Tiger fixes this, realizing that I just typed it in as part of FUS. Yay.
Honestly I can't think of anything else that's a visible improvement to me, other than general speed increases. So underwhelming at this point that we haven't bothered to order Tiger for the iBook. Panther is still great on it.