VaKo hit it on the head. I'd rather mirror ports, but building a tap isn't that difficult. If a comlete n00b like me can build one, anyone can.
It can be worth it. Although I would not recommend Snort per-say. The newest version of Snort (bless sourceforge goodness) is rather interesting to install/configure. If you want quick and easy, jump over and grab EasyIDS. Super simple to set up and you can attach Aanval for streaming it live on a second or third monitor. It doesn't take a heavy trodden hardware beast to run it either.