Someone made me a box type cutter like that and it's cool. But honestly, for that much money I would probably just buy a mitre box from the hardware store. That's what I have and it works well. When you have a box type cutter like that you are locked into a certain size bar. If you change your bar size you can't use the cutter. Also, I'm pretty sure I got my wavy cutter and my straight cutter for cheaper than on that site. Let me check around (of course it could just be that prices have gone up).
It actually has an adjustable stopper that lets you cut soap thicknesses between 3/4" and 2 1/2" in 1/4" increments, which is why I was attracted to it - if I decided to make a thicker or thinner cut, I could! But I get what you say about being confined by the size of the box. I actually emailed them a few months ago, because my bars are 3 1/2 by 2 1/2 and they said they could make it wider for an additional fee. (I think it was $15?) But, like you said, it's already pretty pricey as is.
Have you used a miter box to cut soap? I'm guessing probably if you're recommending it.
I'm just afraid without having a stop at the end that I will still end up with wonky looking bars if the whole loaf accidentally moves forward while I'm cutting. Perhaps that's an unfounded concern?