Been using MC18 since public release, and with music and Blu-ray playback it's mostly stable. Have experienced a few crashes while playing music; one where I hovered over the Audio Path-button, and another few crashes seemingly unprovoked.
Blu-ray playback appears rock solid - no issues so far, indeed very satisfying. The 2-channel sound(JRSS v2.0 downmixing) here is louder compared to MC17 with the same (external) volume setting, which is fine by me, because earlier it was a bit on the low-ish side. The higher output level doens't appear to affect clipping in a more pronounced manner compared to MC17, but I could be wrong - have only done little "testing" here. Will try out internal MC volume, as recommended by Matt, to address the instances where Blu-ray playback can result in clipping(in dynamic/loud bass passages on some discs).
(limited)Library management for music is faultless here.
Overall, for a strictly "meat and potatoes" use with only music(sans DSP) and 2-channel Blu-ray/DVD playback MC18 is close-to stable on my 64-bit, Asus/Intel, Windows 7-based HTPC. I'm thinking of adding a BIG(!) 18" pro driver-based DIY sub at a later time involving an as of yet un-decided amount of DSP usage and maybe that'll provoke issues, but that's another matter perhaps for a separat thread..
Thanks for an already mostly well functioning MC18 program, folks.