I'm absolutely in love with Prism. It's like being set free.
See, the trick here, the main trick, is not running sites in separate, stripped-down windows. Rather, it's that each of those is a separate process, and these processes don't step on each other's feet. They're very light on memory, and if they do have all the Firefox 2 memory leak issues, I'll never know, because I have no reason to keep them open for extended times -- since they load incredibly fast.
The funny thing here is -- many people list tabbed browsing as the major Firefox innovation, or at least one of the top few. What we usually don't think about is how much that became a cage. Since all the sites I'm browsing run in the same process, any memory leaks or crashes become a big problem. I don't know about everybody else, but at least myself, and a few other "power users" I know, tend to leave 10+ tabs open all the time. When some of those tabs are heavyweight web apps like GMail... everything can get very slow very fast.
When I installed Prism, the plan was to leave GMail open on Prism all the time, so if I want to restart the browser, it won't affect my mail. But see... I don't need to have GMail open all the time. I did, because it takes a while to load, and because it was a bit inconvenient to get some new mail notification that integrated well. (If GMail runs in a tab, how am I going to know if it's already open or not?) Now I don't have it open all the time anymore, and I have XFCE's mail notification plugin on my panel, which launches the GMail webapp on Prism. It's brilliant; the speed and flexibility of a desktop app.
Absolutely worth trying.