I bought two recently, both the long and the 14" one. LOVE the 14" one, it's all I use.
In the past, I've tried the Gypsy Gripper, but did not care for them at all and gave them away. I've noticed that the LQS no longer uses them, either.
This truly is a product that works as it is demonstrated. I'm cutting strips and squares using this and a rotary cuttern now rather than getting my Accuquilt out. I've cut as many as 8 layers with NO moving.
If your mat is not extremely accurate, you do have to use in conjunction with an acrylic ruler. I have a Martelli mat, so know that the lines on it are as accurate as on my rulers. As a result, I usually do not use the ruler when using the Slide Lock.
There is a group buy group on Yahoo called JOSews; that is where I purchased mine. I believe the moderator might be doing another Slide Lock purchase soon.