I installed it (family pack) on a Mac Mini 1.83 duo and a Macbook 2.0 duo, both with 2 GB of RAM. I did a straight upgrade, and it's definitely faster on the Mini (I haven't played around on the MacBook much).
In particular, I've noticed that Windows runs a lot faster in Fusion, with the Windows moving smoothly -- without the redraw effect there was yesterday. I also checked Fusion's CPU usage, and it hovers between 1 and 2 percent. Very nice.
I will say that special visual effects aren't great on a 950, but the first thing I do is change the dock preference to scale effect because I like things fast. Quick look is quite speedy, and I expect it to save me a lot of time.
To those with slowdowns despite 2 GB RAM and low CPU usage, check out your available hard drive space. Leopard requires 9 GB!!! For a 80-gig drive that's 75% full, installing Leopard might be enough to slow everything down just because a drive that's 90% full is VERY slow, especially after spotlight takes some more space to make its database.
This is particularly a problem when you archive and install, because then you have two copies of the system and all system applications, which just takes up more space. I would suggest just doing a straight upgrade. It worked great for me on two computers.