The hard part is dealing with Twitter's API rate limit and scaling. It's nearly impossible to pull down the data for large accounts like @aplusk. 5k id's at a time, pulled synchronously (due to Twitter's API paging system), by the time you're able to get all the data, he's already received 1k new followers.
This may seem nit-picky, but the main cool-factor of this is to see which high-profile people follow each other, which it almost works for.
http://www.justunfollow.com provides many other views:
1) Tweeps you're following but not following you back. 2) Tweeps following you but you're not following back. 3) Tweeps who recently unfollowed you. 4) Tweeps who recently followed you. 5) Tweeps you're following but haven't tweeted for the past 1 month. 6) All the tweeps you're following.
