I used the plastic green astro turf type outdoor carpet for a FasTrack layout I did. I really like how it came out.
I personally despise that plastic astroturf stuff. It looks terrible, falls apart, "flakes" come off, etc. I've used a very fine green out/in carpet and my current layout uses grey in/out carpet. I prefer the grey. I think it looks cleaner, a bit brighter, neater etc.