This is another one of those posts that might ruffle a feather or two...let me just say that this is from my own personal experience and observation as a representational artist and may not apply to those pursuing a non-representational style.
I've talked to a number of people who paint who consider themselves "self-taught." If you are one of these folks and wish to increase your skills as an artist, (IMO) you might do well to take some classes, workshops, or a structured online course to learn some of the finer points of your medium and approach.
Why?
Basically, most self-taught artists "don't know what they don't know." There is a wealth of information a gifted artist/teacher can pass on to you in a very short amount of time. This knowledge and skill transfer can help you progress more in a weekend workshop than 2-3 years (or more) of self-study could give you. You have that instructor right there, in the flesh, telling and showing you better or different ways of doing things, helping you through a roadblock, helping you to see or mix color, and on and on.
Another interesting thing that I've noted, besides the quick progress of my work after a workshop, is that self-study becomes easier. I get more out of the books I read and DVDs I watch after taking a class. For me it's a positive feedback loop - the more I study with a teacher, the more I learn from ALL methods.
Are there exceptions to this? Of course, but they are quite rare. The vast majority of the "top-tier" and "middle-tier" representational artists I know have had either a formal art education or have taken several workshops - and some have done both. Some have studied under a mentor or had one-on-one tutelage. But the vast majority have learned important skills from someone else.
Besides artistic technique, you can learn some very valuable "business of art" skills when studying under a professional artist. You'll have a lot of time to ask how he/she got that first gallery representation, or what frames they use, or what galleries treat their artists well, etc. Practically anything art-business is fair game during a workshop, so don't be shy about asking!
Can you study on your own and be a "successful" artist? Possibly, but it's much easier and you'll most likely see quicker progress if you occasionally take a workshop or class.
