Just remember that it doesn't replace your landline, since they don't do proper emergency calls.
There's also no real guarantee of the service, but due to the way the system works, it's inherently quite reliable.
A headset will set you back about �10, give or take. You can get ones like they use in call centres, with regular headphones and microphone attached (which is better than just using your normal PC speakers to hear your friend talking, since that can cause feedback issues with your microphone).