If there is a feature in HTML 5 would be useful for the project you are working on, then it is worth considering using it.
You will need to weigh the benefits against the possibility of the specification changing, the less mature testing tools available for HTML5 validation and the probability that you will need to use JavaScript to fake support in older browsers (of course, the HTML 5 spec gives a roadmap that someone else may have already followed in writing such JS).
If HTML 4.01 gives you everything you need — stick with that.
