I purchased an 18x18 mat at a LQS and can't imagine not having one. I paid a premium for it, however - I think it was ~50 bucks. I'd just started my first quilt and was in the "Ohhhhhh wow, look at that!" newbie phase.
At the time, I didn't realize there were other options. However, my mat has been worth every penny, IMHO.
Complaints: a) I wish I had discovered the link below and picked up a big piece and split it w/a friend instead of paying 50.00 for a fraction of what I'd have gotten if I'd purchased a 60" wide yard. :P
b) my 18x18 piece isn't very big when it comes to pressing seams while sewing garments, esp pants. Or, pressing quilt tops.
The great thing about these felt mats are the firmness underneath when you're pressing. I have a great pad on my ironing board, but it won't hold a candle to the 3/4" thick felt. The thickness is where you come out a winner (again, my .02) because it creates a very FIRM base on which to press.
While I have a very nice ironing board pad, this mat delivers a premium surface for ironing all those pesky seams when piecing - mine are pristine, regardless of my "obvious beginner" piecing. lol
I actually used the mat on top of my kitchen island (formica surface) - and was amazed at the moisture collecting under the mat from using steam when pressing. Didn't hurt my counter (though I'd not recommend doing this!!!) - i was lucky. Just keep in mind, you'll want to let it breathe.
Now, I am in the process of designing a surface to fit atop a table for use as a large cuttong/ironing table in my "soon to be" sewing room. This company offers felt in various thicknesses and its product is 100% wool.
http://www.thefeltcompany.com/sae-pressed-wool-felt/
With a large piece, I can place it on my work surface (or set aside) when quilting and sewing. Why such a large piece? For me, and mind you, I'm a total newbie quilter, when trying to iron the top of my first quilt, it wasn't easy - I want a surface where, as I press, I can roll my top onto a large tube as I complete the ironing (so it doesn't wrinkle).
Hopefully in the coming weeks, as I get my table/sewing room put together, I can share some images!