The PCM-D100 is a wonderful recorder. The quality of audio recordings is much higher quality than, e.g., the PCM M10 (also a great recorder). Very quiet preamps. Very good sound. Works well with built-in mics, plug-in mics, or mics via something like the MixPre-D. I have not heard a portable recorder (in anywhere near the same price range) with the same quality of sound.
However, if you are recording only loud music (as probably do most people on this forum) then the D100 may not be worth the price for you. With really loud sounds, the "quiet" preamps mean much less.
On the other hand, the more you record quiet music, audio interviews, ambient sounds: that's where the D100 really shines due to the high quality pre-amps.
Transom.org recently ran a review of this recorder. Check them out.