I'll agree with JimJS on KNT12.  I have one as well and initially it was the lower cost the drew me to it.  It's decent material, but it is designed to pitch on the diagonal, making it a bit funky and it just feels smaller to operate under.  It is great to pitch your tent under though.  I seldom use it anymore and like the majority here, prefer a flat rectangular tarp. 

I always take one or two depending on group size.  More than 4 under a 10x12 starts feeling crowded, especially if you're cooking under it.  I picked up a Crazy Creek sil-nylon 10x12 a few years ago and it's my go to tarp.  It's not 'quite' flat, as it has somewhat gabled ends, but I like it.  The Sil-nylon is the only way to go IMO.  Water beads off it it like a freshly waxed car and if you bring along a synthetic car shammy (can you say Sham-Wow!) it dries it off super fast and you don't end up packing a soaking wet tarp when you move on.  That shammy is also my go-to camp towel - no waiting for a soggy wet cotton one to dry out.  Forget trying to roll up a sil-nylon tarp too - way to slippery - just stuff it in the bag! 

pd

Actually, Tyvek, relative to silnylon, is pretty heavy and bulky (you can't just compress and stuff it into a small sack like you can with silnylon.  I also understand that it breaks down fairly quickly when exposed to sunlight.  It is more useful to go under you (tent footprint) than over you (tarp.)

I have used the same shape of tarp for years.  The 12' is tight for 4 adults in a driving rain storm but it works.  If you get a really bad storm put your canoe on its side on one side to block the wind and rain, it ialso make a great place to store your gear.  If it does not have one glue a small loop in the middle so you can hang a candle lantern.  If you get Eureka's 8 foot 4 piece poles (very light) you can use 3 sections to keep the tarp low in a storm the 8' works great otherwise.