I've been working with my PCM-D100 over the past few days and must say that the experience has been mostly positive. If, however, you're opting for something truly challenging, go straight to number 5 below.
1. The PCM-D100 has ultra-quiet preamps. And when I say "quiet," I do mean it. Not only are its internal mics quiet, but connecting dynamic microphones to it via an XLR-to-mini cable results in superb audio recordings. Beyerdynamic M58 and M99 shine when paired with the D100 -- I wasn't expecting that with the former. With dynamic mics only my USBPre2 can surpass the D100. The D100 is, without a doubt, a definite improvement over the D50 in terms of noise and overall audio representation.
2. The use of the windscreen is a must. Well, what can I say? That was also the case with the D50 which I didn't own but saw in action several times. Even small movements of the D100 in a room results in air coming into contact with the internal mics and audio gets distorted. The included windscreen is a top performer, though, blocking that effect without muffling high frequencies.
3. If internal mics are plastic, the metal cage is quite tough -- like the rest of the recorder. In short, damaging the mics isn't an easy job as they are properly guarded by a metal cage. The D100 isn't exactly light -- like the M10, but the metal case looks and feels quite professional in hand and in the field. Were I a little too unfair, I'd say the D100 chassis blows the mostly metal Olympus LS-100 out of water. The D100 might not totally do that when it comes to looks, but it achieves such a feat when it comes to sonic features, noise and ease of functionality.
4. Perhaps the best internal mics on a portable recorder? I say "perhaps" because I've had the chance to own the M10, the LS-100 and the D100 so far and have worked with the H4N, the H2N, the H6 and the D50 over the past couple of years. Very simply put, the D100's internal mics generate a well-balanced audio, with highs, mids and lows reflected quite neutrally. Unlike, say, the H6, its mics aren't sibilant at all. Neal Ewers -- whose PCM-D100 podcast can be found in this same thread -- also confirms my point about sibilance.
5. Headphones -- what to use? I'm confused by what seems to be a strange decision on Sony's part. Now here comes the irksome problem. I'm using Senal SMH-1000 headphones --
http://www.bhphotovideo.com/c/product/755659-REG/Senal_SMH_1000_STUDIO_MONITOR_HEADPHONES.html . For a few days I was thinking that I can't get up close to the D100's mics -- internal or external -- as the audio would be noticeably distorted. However, the distortion goes away if you listen to your recordings on the computer. It's definitely there when monitoring via headphones -- regardless of the headphone volume, and is really annoying. Neal also confirms that he can duplicate this weird effect with a few headphone pairs other than the famous Sony MDR-7506. So Sony has apparently done something to the D100 headphone preamps in favor of its own headphones -- at least the 7506.
Now if monitoring via something like the SMH-1000 has become very difficult, the question is which headphones you recommend to take care of the problem? I don't like the 7506 due to its long, non-removable cable. As far as I can tell, Sony doesn't recommend any specific headphones for the D100 on its regional websites, leaving the field open to all sorts of interpretations. I'm currently leaning toward Beyerdynamic T51P --
http://www.amazon.com/Beyerdynamic-T51P-Tesla-Headphone-Silver/dp/B00ELAIHDM , but can't test the thing before purchasing it. Once purchased, it can't be returned -- as strange as that might sound. So have you tested that one with the D100 for monitoring? I'd appreciate your suggestions and D100-specific headphone recommendations.