Having spent some time reading reviews of the DM-50, and correlating this to the DM-100, I'm going to toss this out. If it doesn't make sense, say so.
The DM-50 and DM-100 are "Directional Microphones", that have dual-mode operation; shotgun AND stereo. The DM-100 has options to pull in 90deg and 120deg field of audio, in addition to shotgun mode. In other words, it's not as likely to be as directional as a mono Rode VideoMic, or as wide as the stereo version of the Rode, but gives two modes of operation in one mic. EDIT: Just got in the DM-100, it actually has 4 modes of operation, it has stereo, 120deg, 90deg, and shotgun. Initial tests show it picks up quite a fair bit from behind, and some controls noise (hitting the zoom button to zoom side picks up with a click), will post separately in a DM-100 specific thread.
The DM-100 fits on the camera, cleanly, because it's shorter. Again, this is a compromise between directionality and stereo; it's not as short as a dedicated stereo mic, but not as long as a dedicated shotgun mic.
The DM-100 cannot be detached to a boom pole, and it appears it sends audio thru the hotshoe(?). Assuming this is the case, one can't use it in mono mode, and use a second mic for the right channel. (I.e. interviewing a subject, with a mic on them, and a mic on yourself.)
Given Canon's tendency to mark things up, but also to provide a reasonable product, this is probably a $50 stereo mic and a $50 shotgun mic in a single package. Not great at either, but enough for the amateur who isn't using a boom pole, minimal or no post-processing of audio, and wants to own one mic that they can use as a universal (indoor/outdoor) mic.
The power draw of any mic seems to be a minimal issue, as most are rated for very high numbers of hours off a AA or 9v battery, and nobody seems to be able to measure a difference in battery life with a Canon hot-shoe mic.
This is entirely subjective, I have yet to find a comparative review, or even a good review of the DM-100 (or DM-50) in terms of fidelity, and I don't (yet) own any of the mic's listed. That said, I'll probably purchase the DM-100, and may later add one of the Rode mic's if I desire better quality for specific types of shooting. Despite the compromises they represent, the compactness, versatility and simplicity of power/cables are all wonderful for a 'first mic'.
If anyone has first-hand experience with the DM-50 or DM-100 in relation to the other mic's, I'd love to hear it! (Especially if your opinion differs from my speculation.)