I've just purchased a MX121. Here are my reasons:
1) As a rule, I keep my TV's for 10 years. I currently have a 3.5 year old 65" Panasonic 1080p plasma. 4K would be nice, but it can wait.
2) The MX121 has a passthru mode, so I can override the internal DAC for music with the "latest and greatest" DAC's that might come out.
3) Dolby DTS and TrueHD are more than sufficient. I'm more interested in the music capabilities than the movie capabilities
4) Even if I wanted to I'm not going to be able to take advantage of Atmos - no way is the wife going to allow more than 5 speakers in the living room.
5) It's going to look nice next to the rest of the Mac gear
What's the downside?
1) I've probably not gained any improvement movie/surround sound wise over my $300 Marantz AV8003 processor. The difference in price is painful, but the AV8003 was starting to stick out like a sore thumb. I've always purchased Mac based on quality, with looks taking second placing, but this time I think things are the other way around.
2) Compared to my other Mac gear, the MX121 price is going to continue to plummet. (At the weekend I sold a MC2155 for 20% more than I purchased it for 3 years ago)
I'm going to be very interested to see how the internal MX121 DAC compares to the D100. Also with a C2300 in the mix, it's going to make for some interesting combinations and comparisons.