I installed Tiger on my Mini the evening it came out. I do notice a overall speed difference but it's not Jaw Dropping, but there. Safari 2.0 is very nice and FAST, Love Spotlight, Dashboard is very handy. Networking especially with Windows computers is improved, even on the Mini certain things related to video like Expose' feel faster. One thing though, make sure you have at least 512Megs RAM or stick with Panther. With especially Dashboard running, it wants RAM. I have 1 Gig and it's very smooth. 512 should be fine for most uses.
The difference in speed is as I said not Amazing, but the more you use it the more you will see it here and there. I am very happy I upgraded to Tiger.