I do use and like tabs, but they tend to have better window manager integration, as they are more or less a way to consolidate the "old" multiple window paradigm into a different interface, so for example I can cycle through my Firefox Tabs with Ctrl+Tab. For me they streamline the "usual" mode of browsing, but slow down other tasks, e.g., if I want to display something side by side I have to first pop them out into windows and let the WM handle it.

But what Google is introducing seems to be similar to their Google Talk "windows" that are already available in Gmail. They come in two modes: Either I get a small Ajax-y popup on the bottom right of my browser window, which I cannot resize, move or see when Alt+Tabbing/Ctrl+Tabbing. I absolutely loathe this mode. The second mode is where they pop out an actual window. I like this mode, as I can interact with the chat window as with every other window on my desktop. My assessment above is based on this experience.