These are becoming more and more popular. They provide almost the same level of performance as desktop computers, but not the same quality of video and keyboard. For many people portability is worth the higher price, lower quality video, and lower quality keyboard. The computer can be used for everything from taking notes in class to giving presentations. It is also useful when involved in-group projects since finding a place to both meet as a group and have access to a computer is difficult.
Power consumption is a big factor with portables.
Pentium machines tend to consume more power, which leads to a decrease in battery life. This is not important, if you will always be near an electrical outlet. The 'mobile' processor varies it's speed and power consumption based on power source and battery life. Stay away from dual scan passive matrix color displays. This will allow the unit to be hooked up to an external display for use in classrooms.