I demoed the MX121 while I was in the process of getting my power amp, thinking I could kill 2 birds with one stone. The multichannel processor was superb and I found the user interface fairly intuitive, but I didn't care for how it handled 2 channel. To me, it seemed very "left and right," with little stereo soundstage.
Because 2 channel was my primary focus, I went with the superb C2300 (thence to a C50) and use an older MAC-3 5.1 unit via the pass through when I need multichannel. It's a compromise that has worked well for my needs.
So, it depends on how you're going to use it. If you are primarily going for multichannel, it's a solid contender. Built like a tank, flexible and fully featured.
Good luck & happy listening!