Favorite new toy is the Slidelock
 

For the first time I have absolutely perfectly straight lines WOF! There are probably a lot of videos on it but here is the one I used - ok well it wont let me add a link (or I am not smart enough) - anyway you can search Quilters Slidelock :-)

I have been using one for years. I bought it when it was only sold in Australia and called Rule Steady. I bought it to use as a safety tool for my ruler after I had a rotary accident. I don't use a rotary cutter without it. I have the small and large one now. The yellow ones are good if you use it for cutting. I put it on top of my ruler to hold the ruler in place when I cut.

I have gone to Quilter select rulers they hold the fabric in place for really nice straight line. Best rules I have ever had No needed handles to get the right cuts.

I use Quilter's Select rulers too but still use the Slidelock for a safety tool on top of them. One rotary cut accident was all it took for me to never want it to happen again.

I too, am a huge fan of Quilter’s Select rulers. As my old ones need replacing, I purchase the QS size I need.

Ahh, yes! You are using it the correct way...place on top of your ruler. When they first "came out" people would mark with a ruler, then replace the ruler with the Slidelock to cut. A very handy tool!

Now I have switched to the Creative grids rulers because I'm cutting small pieces and so use a 3 1/2 x 12" most often.

Looks interesting. But since it has a ruler made into it, why do you need to place it on top of another ruler to cut? Or is that only if you need a longer or wider ruler than what is attached? Maybe I don't understand how it is used...

As I posted, I use it as a safety tool on top of the ruler not to cut with it. That was how it was first designed to be used. The ruler isn't moving anywhere not even a fraction and there is no way a rotary cutter can reach my fingers. I will not trust me being careful not to get cut again. Once was enough for me. I have the Dragon Skin slide locks too. I use them for safety.

What Onebyone said. One other reason is you won't (or less likely to)
damage the slidelock ruler if you put it over another ruler. I never use
the slidelock by itself...always on top of another ruler. Whatever works
for you I suppose. Cheaper to replace a ruler than the slidelock. IMHO.

Another bonus use for the long Slidelock is it smooths the quilt sandwich perfectly when spray or glue basting. Another poster here posted that and I tried it. Works great! So much better then using my hand.