Southern4sure: so far we've had Vonage since December and I haven't had any problems. The sound clarity is astounding - it was better than our landline from SBC. Like I've said, the only time it screws up is when cable goes down, but we have our broadband from Roadrunner and they very rarely go down. So I'm quite happy with them.
InstImpres: Yes, broadband phones comes with 3 way, call waiting, call forwarding, caller ID, etc. What I like most is that you can check your voicemails thru your computer. You just log in to the site and your voicemails are recorded as .wav files that you can then listen to or store for as long as you like.
drakethib: I don't know about the security system, but you can disconnect your existing phone line from outside to the inside of your house and then reroute the line to the adapter, therefore you can use multiple phones thruout the house. There's detailed instructions on how to do this on the help pages.
Or you can use those multi-handset phone systems.
PS: I also have a fax machine hooked up and it works just fine.