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.