While I totally agree with everyone here, I just went from 3 different road conversions to a single track frame. i think road conversions are a lot of fun, but a track frame is much more gratifying for the fixie experience. The problem with road frames is that the bottom bracket is low, and since you can't keep the cranks horizontal in the corners, you can never get that deep in the turns. This isn't a big deal, but after striking the pedals a few times, you'll start to get a little nervous.

I think the IROs are great frames to start out with. It has more relaxed geometry but also the nice high bottom bracket. They're not that expensive either!