The XM radio option on the '05 is just that, an option. I was quoted ~$500 plus installation, which would have put this far out the price I would pay. It looked like a pretty involved installation. XM radio contract is ~$10 per month, but I believe Honda may give a couple free months to start.
You can purchase XM radio receivers for less that $100 with a car kit. It is not as elegant as a built in unit, but at the price, I would not justify the built in.
One other note, I understand XM requires a special antenna. The Honda OEM installation includes this antenna. An aftermarket unit would have to be installed.