I have an 8.5x11.5 area rug over berber. I tried an inexpensive mat thinking maybe I could get the job done with $35. The mat turned out to be a complete waste--the rug continued to wrinkle and I was often needing to move the couch, roll up the rug, unroll, smooth it out, repeat...DON'T WASTE YOUR TIME!
I found Ulitmat by Jade Industries online. It was more costly, but I needed something that would WORK. When it arrived I laid the "rough" side on the berber and put the rug on the rubbery side. I marked the shape of the rug, offset by about 1", on the rubber side of the mat. This is important b/c the rug may not be perfectly rectangular. I cut along the markings and put the mat and rug into position. With the scraps I cut pieces to go under the feet of my sectional, rough side on the rug & rubber side contacting the feet. Now my rug stays flat AND the couch stays in place! BONUS!
TIP: buy Ultimat oversized and cut it to w/in 1" of the edge of your rug to avoid a "step-effect."

this worked out very well. Delivered quickly and easy to cut and install.
Great!

With so many mats available out there its easy to try to save on the mat, especially when buying a rug that already costs a pretty penny! But I'm so happy I decided to go for this mat as I keeps my beautiful rug perfectly flat and fixed on the bare floor. I am so pleased that I ordered the same product in an even larger size for my living room rug. I believe it will help keep my rugs looking great for many more years!!

I tried everything to try to get my rug to not slip on very small-loop berber carpet that is slippery; various rug pads from other stores, double-sided tape, etc., didn't work. I decided to buy this based on another reviewer's similar experience, and yay! It works! It's been 4 days now and no moving or tugging or bunching! I really didn't think anything would work. Thanks.