Although I was mostly unimpressed with SSD performance on desktops with plenty of RAM, I have found that they make the performance and battery life of laptop and netbooks wonderful. The 2.5" HDDs in those devices are dog slow, which makes them the bottleneck the majority of the time. By switching to cheap SSD you are able to great increase performance, reduce heat, and reduce power consumption.
However, since SSDs of high capacity are expensive you will likely need an external drive to supplement your netbook or laptop, depending on your use of it. You'll also have to be rigorous about your file management. Utilities like the freeware MoveAndLink can help tremendously.