I'm investigating purchasing an external monitor, so I plugged my 650d into one of my desktop 1080p monitors to see what it was like.
(I apologize if any of this is covered already - I've read this thread in dribs and drabs over the last long while and don't remember any issues with HDMI, and the forum offers no way to search specific threads so I keep finding HDMI issues relating to the 5dIII)
Here's what I found:
1) Canon preview mode gives incorrect framing. Greyscale ML and HaCkED don't appear to work.
2) Focus peaking - a bar of peaking dots appears across the bottom third of the screen independent of what's in focus
3) covering the light sensor to activate arrow keys stops working
4) RAW playback isn't full screen and gives random pink/green/blue/red noise in the bars above&below.
Please take this as a bug report not a complaint.
I'm thinking I'll still spring for the monitor since focus peaking isn't really critical once your screen is larger, and I might be able to make a custom framing cropmark to frame the image properly (addressing #1). #3 is just a mild annoyance.
Cheers,
Dave