Both work and home phone numbers are Google Voice accounts.
I was an AT&T customer for basically my whole life. I always kept my landline because I sometimes did phone interviews where I recorded the calls using high-quality (i.e. expensive) equipment. When I started using DSL in 1999, I subscribed to AT&T.
When I started Blue Oxen Associates in 2002, I decided to get Vonage. I switched to the minimal phone plan for AT&T, basically using it for incoming only, and I used Vonage for all outgoing calls. I paid a flat monthly with cheap international rates. The only reason I didn't get rid of my AT&T phone plan was that I was required to have one in order to have DSL.
On April 8, 2010, I canceled my AT&T service and switched to Sonic.net's Fusion service on Jason Cook's recommendation. I hated AT&T for its outages and horrific service. Fusion was completely separate from AT&T and did not require a phone plan. However, Fusion was slightly slower than AT&T, a worthy tradeoff.
Between 2008-2010, Vonage started suffering some serious quality problems. I looked at alternatives, but they were significantly more expensive or inconvenient, and eventually, the quality problems seemed to work themselves out.
In 2011, Sonic.net changed its Fusion plan to include a phone plan and much higher speeds (20Mbps). The plan cost $5 more per month, but I also would have to pay about $11/month in utility taxes. I decided to upgrade, transfer my Vonage number to Google Voice, and buy an OBi device to use Google Voice with my regular phones. I went from paying $60/month ($35 for Fusion + $25 for Vonage) to $51/month (for Fusion) plus a one-time cost of $50 for the OBi hardware. This gets me off Vonage, gets me considerably higher bandwidth, and I get a regular landline, which will be higher quality than Vonage was. I can use Google Voice (via OBi) for international calls, whose rates are cheaper than Vonage's anyway. After five months, I make up the cost of the OBi, and I save $9/month.
September 2011: Upgraded to a Motorola Droid 3 on Verizon.