Two questions

[nachoman]

My experience dates from when I was a kid (late 1980s).

I remember having both brass and NS model power flex, and atlas NS (plus a hodgepodge of sectional track). From what I remember, I had no issues mating Atlas to MP flex, and the appearance was similar. I remember the MP NS being a little bit stiffer than Atlas to flex, and the color of the rail wasn't as shiny as the Atlas. Looking back, I am wondering if that MP NS was actually steel back then - but I don't remember having problems soldering to it. As for how it ran - I had no issues. Any derailments I had were more likely do to cheap wheels and trucks, sharp curves, underweighted cars, cheap turnouts, and bad vertical transitions and poor rail joints.

I am not using commercial flextrack on my current layout. If I was, I would consider appearance over all else and probably choose to go with code 83 (I hear MP makes that now, too). But if cost was an issue, or if I had a lot of hidden track, I would probably go with MP code 100, and use turnouts from Peco or Atlas. Weathering track does a lot to help appearance.

BTW, if you decide to go with MP, I encourage you to post a review on this site. This question comes up from time to time.