I dumped my landline, but do have a cellphone which acts as a back-up in the rare instances of my cable connection going down (2 incidents in 2 years, for a total of around 15 hours).
$25 for unlimited local, LD and Canada calls, and comes with call waiting, call forarding, voicemail and the ability to save voicemails to my computer as WAV files.
Also available, but which I have't used, is virtual phone numbers. For $5/month you can get a phone number almost anywhere in the USA (Mexico City and London coming soon, if not already available), letting people in those areas (like relatives) call a local number to reach you LD.
I have a cable connection as it's slightly faster than typical DSL, so sound quality might be slightly better too.
Vonage's main competitor is probably Verizon's VoiceWing which costs $5/month more than Vonage. Either one will save you money over your landline local/LD anyway....