(my old mat, I still love it and will use it a lot.)
I had been using Gaiam mats for years (I still LOVE and use this bag from them). There is nothing bad about Gaiam mats, they come in a great variety of colors and designs and for most people they are perfect. In fact, I have a really cute Gaiam mat that I have used for years that I do not want to get rid of because it is super cute and it works well.
But I was having a problem with the mat being too thin for me, for most people this isn't a problem, but for me I was having an issue with joint pressure during certain poses and while it was easily fixed by layering another mat underneath...I always felt silly at the studio grabbing a mat to put under my....mat.
Then at Christmas, I got a prAna mat. I got the E.C.O. Mat and it is amazing. All my issues with joint pressure in poses are gone, the grip of the mat is incredible, and while it is heavier (because it is thicker) I don't mind carrying around the extra weight because the mat is just what I need.
The great thing is, if you are new to yoga, a lot of studios have mats you can use. I suggest doing this before investing in a mat. You might find that you prefer a thinner mat or you may find, that like me, you need a little extra padding.