I've been doing them for about 15 years. I was doing about 16-20 shows a year, and made enough to pay the bills. My sales were good enough to justify trips to Florida, Chicago, St Louis, NY, etc (from the DC area). Last year, the bottom dropped out. It's a very tough economy to sell in. It's also very competitive. When I started, there was probably a 30-40% chance of acceptance. With the proliferation of d*****l shooters, the amount of applicants has increased by a lot, especially at the better shows. The initial expenses are high. A good booth, and display panels, can run over 2K. Booth fees of $500 (for a weekend) are not uncommon. Add hotel/food/gas/materials costs, and a 2 day show can reach a break-even point of 2K.
If you're just starting out, consider break-even a good goal. You are up against people who have been doing it for years. Their sales skills are honed, their booths are expensive (and professional looking), so they'll have a few advantages over you.
If it's something you'd like to pursue, I recommend finding a local flea market, or farmers market. Usually you can get a table space for about $20.00. You'll learn to talk about your work, valuable sales techniques, dealing with rejection, and which images people are willing to buy. I did a flea market, in DC, for 2 years before I ventured into the juried shows.
Because of the economy, I decided to cut back this year. I will only be doing about 6-8 shows. I'm only applying to shows where I've had success in the past, and have little, or no, travel expenses.