Sean, your link does not work. Can you please explain, what kind of device you are looking for?
I'm using "the Parallel" from Versalab
see http://www.versalab.com/server/photo/products/parallel.htm This is a simple device that is best suited to check alignment just before making an exposure. It does not require any changes or installations on your enlarger.
the Parallel is basically a laser pointer in a solid, well engineered and calibrated case. Since the case is relatively small, you must make sure that your easel (or any other reference surface) is real flat. Otherwise you will adjust your enlager to wrong conditions. If you take care of that, it works 100% and is very easy to handle.
I second Thilo's evaluation. I didn't like spending this much on something that isn't used that much. but in made alignment of the negative stage, lens stage and easel very simple. I can now make 16x20 with all the corners sharp. my enlarger, an Omega, came from the factory extremely misaligned.
I made my own for about $25 out of a mirror and a Daisy laser scope for an air pistol from Walmart. The scope has axis adjustments for the beam and a nice stable base. The trick is to aim it at the mirror and rotate the scope and adjust it so the reflected dot does not move. Then you can be sure that the beam is perpendicular to the base. Then simply adjust the enlarger until the reflected beam hits the source. Sounds more complicated than it really was. Now my enlargements are nice and sharp - corner to corner.
I've heard great things about the Versalab as well, but I went the cheap route - 2 mirrors and a prism from an old pair of binoculars. One mirror sits on the baseboard or easel, or held against the bottom of the lens. On the second, I scratched a small circle out of the plating. This one goes in where the negative holder normally sits. The reflective sides face each other. The prism sits on the top mirror and allows me to look straight ahead and down through the hole. If everything is aligned, you get that endless barber-shop effect.
I used to align my enlager with mirrors as well prior buying a Versalab Parallel. The main advantage of the Parallel is that you can check alignment in any position of the enlarger head without changing anything. If your enlager is perfectly aligned at one head position, it might need a correction for very large prints with the head in top position, depending on the precision and length of your axis. The Parallel is an "alignment on demand" tool and mirrors are not really an alternative for that.
Thilo, the mirrors also work at any head position. I usually check all stages at the beginning of a major session, and then make quick checks when I move the head. It's the same procedure as the Versalab- a mirror on the bottom of the lens (without changing anything) and another on the baseboard. I don't do enough enlargements past 8x12 from 35mm to justify the $190 for the Versalab.
Either system is light years ahead of spirit levels, rulers and scratching X's on negatives.