It would really depend on what you want to use it for. (My experience is with the CR-48, so I can't speak to the commercial iterations of the ChromeOS notebooks.) I throw the CR-48 in my bag when I'm headed to the lake or on a trip and just need simple email/web access. Its also become great in the kitchen for recipes, in the living room for adding to my Netflix instant queue, and on the boat dock for looking up random stuff (always something that needs to be fixed, examined or researched on a boat dock).

In summary, its a very useful tool, but absolutely not a replacement for my MBP or T400.