I cannot express to you how much I love the Stripology ruler. I have the original rectangular one, but I understand the Stripology Squared is a little taller than the rectangular one, but smaller in width so you can't cut as long a piece. In regards to the usable length of the cutting channel - once I figured out a couple little tricks, I haven't had a problem with not being able to cut a full WOF without much adjusting. I posted about it here:
Stripology Ruler.
Basically, I don't line it up on the very bottom line but instead the top of the 1/4in dashes which is easier for me to see anyway. I use a 45mm cutter instead of a 60mm so I can get closer to the bottom and top of the channel, and you do have to try to get the selvages somewhat close together so if it was wound on the bolt really badly, I try to adjust that before I iron the fabric.
It's a great ruler! I've never had the cutter slip out of the channel, and I've never had it slide while I'm cutting. It's my favorite ruler and I use it every chance I get.