If you’re not familiar with them, RailsCasts are really high quality screencasts focused on Ruby On Rails development. While I don’t do much (if any) RoR development consistently, I think this 10 minute screencast is a great look into why Twitter Bootstrap is so powerful. If you’re a RoR developer it’s even more applicable to you.
Twitter Bootstrap is really a fantastic piece of technology, it highlights just how powerful CSS can be when written just right. If you haven’t built anything with it, I highly suggest you do, it’ll literally take just a few minutes.