Corner Ruler v.s. Slot Style Tools
I think our "Slot Style" tools are a significant design improvement over the more traditional Corner Ruler / Roamer tool style.
The Corner Ruler is like the IBM of the coordinate plotting world. Traditional, maybe even old fashioned. But you can't go wrong with IBM.
The biggest improvement with the Slot Style tool is that one edge of the tool is always placed one a UTM grid line. This simple step ensures that the tool is always square with the grid. With the Corner Ruler you have to eyeball the tool to be square with the grid.
The second biggie it the ability to quickly verify that the tool you are using actually fits the grid. A quick glance up to the top on the northing scale on a slot tool should show the end of the scale aligned with the top of the grid square. Sure you can check this with the Corner Ruler, but it requires a separate placement of the tool on the grid, and this step is usually omitted. When your using a tool with multiple scales on it it is important to do the quick check to make sure you are using the right scale. Plus in these modern times, when the map we are using has likely been custom printed from some sort of digital data source, it's important to check that the map is really printed at the scale you expect it to be.
Are there any downsides to the Slot Style tool?
Aside from bucking tradition, there is only one downside that I have experienced. The Slot Style tools are not quite as rugged as the Corner Ruler Tools. This is because the slot is a bigger hole in the plastic than the hole at the corner of the rulers on a Roamer. That said, I have not had any reports of the slot tools failing in normal field use.