I started out with mats that I purchased at Home Depot - which I think are the same ones that Costco are selling. (My workout space is a basement).
Then my boyfriend surprised me with a set of mats from greatmats.com for my birthday. He got them to cover an 8x8' square area, so the corners of the square are rounded and the borders are smooth and they came in 2 colors (blue and black) to make a checkerboard effect. (I'll post a pic somewhere if you're interested).
But comparing the 2 kinds of mats, I've found that the Home Depot/Costco ones are not as thick as the 'great mats'. I believe the great mats are about 1/8 - 1/4" inch thicker. We had tried to add my old mats in to make the space even larger, but the difference in height would have been a definite tripping hazard.
Also the great mats have a 'squishier' feel to them - bouncier, almost. My dumbells and barbell don't leave a mark on it during the workout, but if I left them on it overnight they do. However, the indent seems to go away by the next day. I also notice an indent left by my step after a workout, but again, it's gone the next morning.
By the way - I put the smoother side down, bumpy side up - I find it grips the cement floor just fine.
I know the great mats are 'much' pricier, but I think they're worth if it your current workout space is a permanent one, and you'll get a lot of use out of them.
Hope that helps...
Sandy