I have used Windows 7 extensively at home for the last few months and can say its agood improvement on XP and a drastic improvement on Vista.
Noticable improvements:
Improved boot up time and automated connectivity.
Diagnoses and resolves technical issues alot easier with the troubleshooter, XP is not helpful at the best of times.
Inceased browsing speed, IE works very fast depending on connection.
Some nice new features
Graphically impressive
Easy to use
The Run command has an active serach function which is really useful for finding things quickly.
More control over devices
Windows 7 centralizes control of all devices. The controls all appear in a place called Device Stage.
Do have an issue with the the Intel Graphics accelerator it kept constantly crashing the graphics driver and blue screening the laptop, updated the Intel graphics driver and the problem does not occur as much as it used to.
I did update Windows 7 onto a Dell D630 Lattitude laptop which is a good spec I would say an older model wont handle Windows 7 very well!?!
Over all I think its excellent and alot faster than Vista or XP!
I will buy a MAC one day though! Im sure of it! ; )