I've done a little with a math competition site called projecteuler.net. I use the term competition very loosely. It's more that you just do problems at your leisure, and the site just happens to keep a leader board. I highly recommend the site, but with this caveat, it's about as addictive as crack; I know that sounds ridiculous, but consider yourself warned!
I've been using the project euler problems as a good way to familiarize myself with Ruby and it definitely seems to have been success in that regard. Although the problems are small enough in most cases that you're not building anything sizable, it gets you familiar with the libraries and gives the opportunity to think ruby thoughts where possible. I haven't run into a performance issue doing the problems in ruby yet. I'm sure I'll run into them at some point, 'tis the nature of the beast, but often that just means there's a better way to approach a problem.
I believe I've broken the top 900 (I guess that's a woohoo moment? ... right?) but if I actually get more than an hour or two to spend on it a week, hopefully I'll manage to finish more problems.
