I installed Lion on both my 2006 Mac Pro and the MacBook Pro, and it worked well enough on both, with some limitations. I could copy and paste text between the virtual and real OS X environments, but not images. (Image copy-and-paste works in both Windows and Linux virtual machines, but not in OS X VMs.) An attempted video chat in iChat didn't work; the iSight camera appeared as a tall thin black box, without any video in it. However, a video chat via FaceTime worked fine, as did a video call in Skype.
Other limitations running a virtualized copy of OS X: You can't use more than one monitor or video acceleration of any sort. However, if you're just looking for a safe place to test software, or to run as a user with none of your typical user's add-ons, a virtualized Lion environment is a great way to get those things done.
Fusion makes it fairly easy to install and run Linux operating systems. You can use run Linux VMs in Fusion's Unity mode, hiding the Linux desktop and making Linux applications windows appear alongside your OS X apps. There's no OpenGL acceleration in Linux virtual machines, though.
VMware also has a collection of nearly 2,000 downloadable virtual appliances (ready-to-run configurations of operating systems and/or applications), many of which are free of charge. After downloading, you can be up and running with these systems with a couple of mouse clicks.
To manage your collection of virtual machines, Fusion 4 provides an updated Virtual Machine Library window. This window makes it easy to see the state of your virtual machines at a glance, and you can Control-click on any virtual machine to resume it, modify its settings, and handle other administrative tasks.
The currently-selected virtual machine is clearly highlighted, so there's never any question as to which machine will be affected by your actions. This window hides itself when you launch a virtual machine, which is nice. It'd be really nice if it then unhid when you closed that virtual machine, but it doesn't.
Fusion is a complex program, but Fusion 4 goes a long way to making that complexity easier to manage. For example, the revised Settings panel (used to set each virtual machine's options) resembles that of System Preferences, and is easy to understand and use.
You may be annoyed to find that you can't move this window around. That's because it's not really a window, but an overlay that's locked onto the virtual machine window. At first, I disliked this feature...until I opened more than one virtual machine at a time. As an overlay, it's completely clear which virtual machine you're modifying; this wouldn't be the case if the window were freely movable.