It's great for some things, but bad for others. If you're using Firefox for example, and the notebook is calculating something, it will lock up all Firefox windows. Also, if you are printing a lot, the html rendering is very slow and consumes a lot of CPU.
Great for previewing graphs and copy pasting and executing things out of order.
The first part regarding Firefox is not true. Calculation is completely asynchronous in the IPython notebooks, i.e. if you run a cell that doesn't output anything your browser is idling.
Even if it has one pretty large output it works quite well. However, if you `print` each intermediate result and have a few hundred of those you get a problem indeed, there is a need for some kind of overflow protection here.
Great for previewing graphs and copy pasting and executing things out of order.