This is my limited experience of D Star so far.
When D Star, the new digital voice mode introduced by Icom became available, I had to experience it, as digital technology is simply amazing. I purchased an Icom ID-800H and installed it in the mobile.
Digital technology offers a better signal to noise compared to analogue, so I wanted to see just how well this particular digital radio would work in a variety of situations, particularly from the mobile.
In the stationary situation receiving another stationary amateur station, D Star has a better weak signal performance than FM, but not always by much. What you notice first with D Star is the audio. All stations sound similar with a slightly nasal sound and this does reduce readability somewhat.
In the mobile situation FM is better than D Star, which disappointed me. When D Star receives a signal with variations in signal strength, such as when you are mobile, the audio breaks up into unintelligible garble. It sounds a lot like the little robot in Star Wars, R2-D2.
This difficulty for D Star to handle weaker mobile situations, where there is signal variations, which is the norm, does limit D Star considerably. If you drive the same path and compare D Star to FM, FM wins. A weak fluttery signal on FM, even though it is noisy with reduced readability, is still readable, D Star is not. And it does take D Star a short while to recover, hence the digital dropout last longer.
However all that said, for me, D Star has a big advantage over FM when it comes to World Wide or local linking. Being digital the audio does not suffer loss of quality as it is linked, be it via radio or the Internet. I sometimes listen to various nets from the USA and the audio is good, considering the amount of linking situations it has to go through to get from the USA to Australia.
D Star unfortunately does suffer from a limitation which is annoying, the need to leave considerable breaks between overs when in QSO via D Star repeaters. The recommended is 4 seconds so all the various linking and or repeater situations work without the R2-D2 breakup. I think the need for the 4 second break is so the digital encoders/decoders have enough time to sort themselves out from one audio source to the next audio source, as the break between overs is a loss of signal situation.