I've gone to the H Street market for several years . Not many vendors compared to Dupont but I've found that they have just about everything in season, and the quality is better than Eastern market. Also Dangerously Delicious pies sells wonderful mini pies, so that I can try a couple of different ones. On the first saturday of each month, Red Apron Butcher shop and the Dolcezza gelato folks are also there.
The H Street Farmers Market is part of the network of Fresh Farm Markets. Their flagship is the Sunday Dupont Circle Market. http://www.freshfarmmarkets.org/
The H Street satellite is smaller but has gotten much bigger this year. It's from 9 to noon on Saturdays which is more convenient for many people. The prices are higher than Eastern Market but there are more interesting purveyors.
The RFK market is terrific but don't expect "designer" products. These are old family farmers from the Delmarva, many from St. Mary's County and east of DC.
I've been shopping there for more than 20 years and love it despite the location under the Metro tracks in the RFK parking lot. At least you can park very close to the vendors. Lots of food stamp shoppers. A fish vendor (you have to gut and scale them yourself because of DC law) and usually a good BBQ stall. The fish vendor usually has soft shell crabs if you get there early, and often has hard shells too - both at excellent prices.
The vendors grow much of the produce themselves, but do augment it with things that they buy from further South to push the seasons or from wholesalers so that they can offer the full range of produce. It's nice to be able to buy lemons and bananas for instance. Saves another trip to the store on a busy Saturday.
This is a particularly useful market if you plan to make jam or can tomatoes. They will sell in bulk and often offer terrific bargains on quantity purchases or close-outs at the end of the day (about 2 pm.
)In general the quality is very good and the value is very high. Probably the lowest prices of any farmers market in town.
I highly recommend the Mexican family for their selection of different varieties of peppers, tomatoes, and eggplants.
I was there (H Street) once, and it's about the size of the Foggy Bottom one on Wednesdays - Small - Whether it's worth your time depends on where you are and what you are looking for - It had a few stalls but still a farily good selection of veggies, meat, and breads (this was last year, I don't know what, if any, has changed). I don't remember buying anything since I hit the Dupont FM on Sundays anyway, so a Saturday market doesn't mean much for me unless I am in the area anyway. I keep meaning to check it out again.