It'll arrive tomorrow or Monday. Haven't had any hands-on experience, but apparently it's a good portrait/landscape camera. AF isn't very useful for moving things, especially if you want to shoot burst (>1fps with AF, 4.3 with single AF lock/MF).
Messed with some RAW files last night, and it's usable up to ISO 3200. After that, colors get funky. Nice detail, though, and interesting color rendition (CCD-like). Also bought a Nikon lens adapter for it. :3 Magic Lantern will significantly help MF shooting with focus peaking.
Probably post a review on my site in a week or two.
Not sure specifically what features are currently implemented (EOS M is in alpha), but for video, even just the ability to up the bitrate is huge. Fast motion scenes and high ISO footage will benefit the most. Especially high ISO, where you can use a program like Neat Video to remove 90% of the noise, without having to worry about it already being compressed to hell by the internal encoder.
Lots of other features are/will be available, like the previously mentioned focus peaking, zebra stripes (shows blown highlights) and false color (shows range of exposure in highs/mids/shadows), uncompressed HDMI output, histogram, and a bunch of audio features. Oh yeah, and HDR video! :AF: Hoping the video quality can come close to my GH2 once most of the features are implemented.
As far as photos, there isn't much as far as things that improve image quality except maybe HDR in a single photo (dual exposures, alternating between lines), but it adds things like an intervalometer, programmable bulb time, and bracketing, along with the focus and exposure aids from video.
Focus peaking is the big thing for me, since it makes manual focusing and especially macro work a snap! I've got some programs on my laptop and tablet that attempt focus peaking, but they can only do so much by interpreting the camera's output. Once you get above ISO 3200 or so, the noise interferes too much for it to be reliable. ML uses the camera's native phase detection, so you're seeing exactly what the camera's seeing when focusing.
Considering it really isn't a permanent hack (reads from the SD card every boot), there's really no reason not to install it. Soooo much added functionality, and more to be added as they keep coming up with ideas/breaking down coding walls.