Are you referring to the MCX MR? Their CSASS offering?
I'm never the first to hear anything but I've not seen any sort of release date and it's not listed at my distributors. I think you're going to have a difficult time finding one this year.
MCX 500? Again not familiar. I've got a 556 that I enjoy shooting. If Sig can deliver on the conversions and some accessories I think it'd be a great option.
I'm a Sig P556/556 fan. The differences of the MCX were not enough (several I did not like) to transition. The MCX MR though is a new beast. I like the steel and replacement part capability though I can't see that I would wearout anything. Thumbs up for going with a single recoil spring. Don't like the dual charging handles (seems like they want AR and HK market share) could toss the and the FWD Asst by using recip bolt handle and put on a heavier barrel if they needed to keep the weight up. All in all I like it. Have not seen anything but prototypes so the production gun may differ when released. I'm sure the MSRP will be high and based on the success of the MCX and hype on the MR, street price will be higher with limited availability.