It really depends on what you are doing. I have found it works just fine for me, but I am not able to have a lot of tabs open; under 10 is good. You will notice a slowdown if any of the pages or apps are dynamic or have heavy scripting. My wife tells me Farmville slows it down pretty heavily. Overall it is a awesome device for browsing, blogging, managing your cloud files, and other things like that; if you are looking to be a "power user" you should test one out before making the purchase.
The newer released Chromebooks are a lot faster than the Cr-48 test notebook, but still are not as fast as a full fledged PC for multi-tasking.