I have a friend who describes riding in these conditions as "cheating death at 8 mph". That might be a bit of a stretch, but it's certainly more difficult than zooming around on clean pavement.
I took advantage of the warm weather and rode in to work rather than gut out another bus ride. I left a little later than usual and avoided Park Ave. (my usual route) due to the ice ruts, sloppy snow and cars in the bike lane. Instead I went north to downtown on Oakland, which was slow going with some clear patches between bumpy ice patches, (with a little mashed potato snow through in to spice things up). Oakland was slow but acceptable up to about 24th or so, then I bailed out and went for Park to cross the freeway. Avoiding Park Ave. was the right idea - the bike lane is there, sort of, but ice ridges are encroaching on it and will catch your wheel if you get too close.
Unfortunately, the happy motorists don't seem to understand the three foot law well in winter, and i got buzzed quite a bit more than I like. The downtown conditions were really very good. The ride home was better; I stuck to Chicago Ave, on the advice of Bikesmith from earlier this year. Chicago was in much better shape than Park/Portland the traffic was only heavy around Lake St.
I was prepared for this to take longer than my fair weather rides, but nevertheless I was surprised at how long it did take to get where I was going. What is normally a 20 - 25 minute ride in good conditions stretched to something like 40 minutes today, it seemed.
Still, it was kind of fun. I only fell down once, which I declare to be victory because I didn't get run over (thankfully it was on a side street this morning and not Park or Portland). The ice ruts are very technical to ride on, and if you are going slow enough, attacking them is a good challenge and would certainly build some skills. My main problem, and the reason I went down, was that I was riding faster than conditions would allow. I figured that out as I picked myself up off of Oakland and remembered that concept the rest of the day.