Home Security Systems?

Anyone know anything about a wireless home security system? I had a system in my last house which was pre-wired while it was built. I loved it, but now live in an older home. There is no access to wire in a system, so it looks like I must install wireless.

I have had some of the usual suspects over for estimates, but the advertised "free system" ends up being over $800, plus $34/month service.

I want to do it myself because the pros seem to be tremendeously overpriced. I can't find a system I think looks to be of good quality. Anyone know of a good system, or a message board where people discuss this as a topic?

Thanks,

mbg