I love teaching adult beginners, partially because I was one (I started as a kid, stopped when I was 11, and started again when I was in my early 30s). This is my note of encouragement for anyone who feels that their life would be more complete by being able to play the violin (or the viola) to take the plunge and start.
It is rewarding right away, as long as you work with a good teacher. The average adult student who comes to me to take a first lesson (with no experience with the violin and no knowledge of how to actually read music) is able to play "Twinkle Twinkle Little Star" at the end of a thirty-minute lesson. An adult with a background in music will be able to "Twinkle" in about 15 minutes.
Children, in my experience, don't learn as quickly as adults, at least at the beginning. Perhaps it is because adults generally have more physical awareness (having had the same sized hands and length of arm for a while) and more of an ability to focus attention on a task at hand. Adults become frustrated more easily, and they tend to be more self-critical, but they also tend to understand the ways and means of solving technical problems, and apply them.
Here are a few guidelines for adult beginners:
Do not buy a $100.00 instrument that "looks good" on ebay. Do not buy a $100.00 instrument from a music shop (on line or in the neighborhood) that specializes in electronic instruments and guitars. The music shop owner might have the same supplier as the ebay shop. These instruments tend to look like violins and have been known to function like violins, but they are difficult to play, are generally poorly made, and are difficult to tune.
If you want to buy an instrument, buy one from a reputable violin shop. There are shops all over the place. A good violin shop will make sure that you have an instrument that plays well and is set up properly. You pay about ten times as much for an instrument from a good violin shop as you would from a guitar shop, but you can be assured that your instrument will serve you well for a long time.
Another option is to rent an instrument from a good violin shop. Just about every violin shop I know has a rental program. Some apply all the money you pay in rent to the purchase price of an instrument. Any shop worth its rosin will provide repair and re-hair service (bows need to be re-haired once in a while). Many shops offer all of their services to people who live far away, and they do much of their business by mail: shipping instruments for purchase, rental, or repair.
Good teachers are more difficult to find than good student violins, but there are good teachers everywhere. Ask around. If you have a string program in your school system, ask the more advanced players about their teachers. Call the teacher who is recommended most often, and if s/he is unable to take you as a student, ask for recommendations.
Don't be fooled by "brands." There are some terrific teachers who teach by using the Suzuki method, which is a method developed for very small children who don't have reading skills (we're talking about 3-year-olds). There are also teachers who use the Suzuki method who are not able to communicate well with adults (or older children, for that matter). There are also people who use Suzuki materials, but do not apply the "method." The quality of a teacher is often quite separate from his or her method.
(Thanks to Michael for this ad from Popular Mechanics)