Maybe for crossing the US/Mexican border, but not on the local buses running up and down the carretera!
Back home, I had my own car and I loved it. For many years I lived in a rural area with no public transport. A car was an absolute necessity. Here, the environment and my lifestyle are different. My husband drives but I don't, and I find that I can get around very well on foot and with local buses. If I was here on my own, I would not have a car. Whether the bus service is good or not depends on what you're accustomed to where you came from. I find the local bus service is just fine and the long-distance bus service is totally awesome. We know several Canadian snowbird couples who fly down and live carless, and they manage quite well without leaning heavily on us for rides.
We took a weekend trip to Guanajuato with friends and opted for the bus because we didn't want the hassle of driving and parking in Guanajuato. Yes, you can sightsee without a car, and you don't have to depend on guided tours if you are a more independent traveller.
But it really does depend on where you live. We saw some lovely homes in Chapala Haciendas, but settled in San Antonio for the local village atmosphere and the convenience of being a 5 min walk from all our daily needs.