The built-in mic is not very useful in most situations. The external optional mic does an okay job if you are close to your subject but you really to define what you will be doing. Unfortunately there is not one type of mic that works for all situations.
we use the Sennheiser ME64 with a Lightwave fuzzy and it works great for run and gun. we also spread it across 2 channels and run one at a lower level than the other to make sure that it doesnt distort. in post it so much nicer to to be working with a good mono mic (at 2 different levels). those stereo on-camera mics dont just sound bad, but the stereo spread can be very difficult to add to the mix.
btw, the on-camera mic on the HVX is much better than most.