Be careful with the fixxer, it's only comptable with hubs that have splines for a Shimano freehub body. Ways to assess this are: remove the axle/cones and insert a 10mm allen wrench in the freehub side. If you can engage it, then the freehub is removable (turn the wrench counterclockwise to unthread it, then pull it off) and it is compatible. If, like some modern hubs, the freewheel mechanism is built half into the freehub body and half into the hub body itself, then it won't be compatible. Most of the ones like this that I have seen have cartridge bearings on the freehub side (the axle is pressed into both sides, and spacers are removable with allen keys), while compatible ones have cup-and-cone bearings. Surly has the tech manual for the Fixxer on their website, so if you don't know what I'm talking about, take a look at what the hub splines should look like there. You should be absolutely certain it will work before plunking $85 on a hunk of aluminum.

It works very well though, I use one for my disc wheel.