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They have the potential to be useful, depending on your needs. For artists they can be a great tool. However, Zojirushi NP-HBC10 they probably will be more of an aggravation for others who might not actually need to use a stylus. In your case, you might want to consider getting a bluetooth keyboard or a cover that includes a keyboard. Either option will help you type faster and more accurately than a stylus.
In my opinion, Bamboo is the standard for a stylus. Perhaps you have been looking at a Bamboo, because they are a little more expensive but you mentioned $30-$40, which would get you one. There are a lot of cheaper options out there if you don't want to spend that much, but for the most part I think you get what you pay for.
If you I like to use it with my tablet when I am sitting at my desk, but that is where it lives. I never take it with me for the reason mentioned in the question - it is one more thing for me to try keep track of, and may track record is not very good in that respect. For the most part I don't consider it necessary, it is more of a luxury. When I am drawing, it does come in handy, so there are times when I have to sketch something that it actually is a useful tool. One thing that I have noticed with different brands of stylus is how different the nib feels moving across a touch screen. There was one I tried (forget the brand) that felt like it was dragging, which I hated. I suggest that you try to test some out to see which you like, but if you have to get one without trying it out, go with a Bamboo stylus.