Kieran

The guide sits one or two millimeters  back, I presume to allow you to see the cutting edge better.
You won't find a better trimmer anywhere - we replaced ours last year after 20 years of heavy professional use and they were still working, just looking a bit tatty and squeaking a bit (like me).

Brian

Kieran

After a few goes you will be able to judge exactly where the cut is - the metal cutting edge is a good guide. You can also line up the back edge of the print with the 8" mark on the side ruler. The metal bit is useful for trimming large numbers of individual prints.

Brian

I also have a Dahle trimmer.  I had the opportunity to look at some Rotatrim units while I was doing my research a few years ago, and went with a Dahle.  I liked the construction of the cutter much better.  The plastic guide bar is set back from the cutting edge on the Dahles also, just like Seth says.  Both the Rotatrim and Dahle cutters will trim hair width pieces off of paper, so I just make my first cut where I know there will still be a little strip of blank paper before the print.  Then I just make little tiny cuts till I have it right.  Once you make the first cut, the rest go easy.

Thicker than what?  The Dahle cuts through poster board, thin backer board and 15 mil inkjet paper easily.  The better Rotatrims will also.

Anything thicker I use my mat cutter with a straight-cut blade.