I'm using CallCentric.com for SIP service and terminating it on an Asterisk/FreePBX system. Handset is a generic Uniden wireless set from Costco connecting through a Linksys PAP2. Absolutely love this setup! I front-end all the incoming calls with an IVR that filters out 99% of the sales calls, and voicemails get sent to me via email where I can listen to them as a .WAV file on my blackberry.
Hardware is an MSI atom-270 based barebones PC (It's their "Wind PC") with a laptop SATA HDD and 2 GB memory. It's running the AsteriskNow 1.5 distro which comes with FreePBX and CentOS. Currently using Cox Cable for internet access and never had a call-quality problem.