I arrived at Union Square around 2 PM and left around 3:30, and things were quite busy with a large number of merchants, so expect to have some degree of hassle in walking around with crowds and having to wait in lines for particularly popular stalls if you happen to be there at a high volume time.
Just about everything that one would expect to see at a Farmers' Market is there and repeated at least once; herb and flower merchants with potted plants, dairies with fresh butter and milk, vegetable and fruit stands, freshly butchered meat, and something that isn't found at farmers' markets everywhere- fish stalls. Even more unusually, these stalls are broken up by the usual Union Square population of street artists selling everything from colourized black and white photographs to painted silk nudes of Japanese warrior women to a few minutes of breakdancing and winning personalities. All in all, it makes for a very diverse experience.
Prices were about what one would expect from a farmers' market, slightly higher than for less local, less fresh produce. The fish prices were the exception, coming in somewhat below what I'd expect to see at a major store. Cutting out the middleman of the supermarket in this respect makes sense, and in my case resulted in a cheap tuna steak.
In the end I spent about $12.00 on the dinner I had last night, which ended up being very tasty. I had a tuna steak which was seared lightly in a sauce made from raw buckwheat honey, five spice, three shots of scotch, and a peeled and sliced peach. To balance, I managed to find, unexpectedly, some okra at the market for a low price since almost no one in the North knows how to prepare it correctly. Anyway I breaded that with cayenne pepper, flour, and salt and fried it up, resulting in a nice spicy side dish to counterbalance the sweet, smokiness of the scotch-peach-honey tuna. All in all it was pretty cheap and simple to prepare, especially for a set of things that would've cost about $25 off of a restaurant menu. Take a look:
The bottom line is that the selection and density of market stalls at the Union Square Greenmarket is pretty extensive and there are definitely unusual things to be found there, given that variety. Walking around for an hour I threw a great meal together, which could probably have satisfied even two people, without having to completely empty my wallet, which was my goal going in.
By comparison to other farmers' markets I've visited this was a very good experience, but I can't compare with other Manhattan markets yet so I'll reserve judgment for now. Still, given its accesibility and the frequency of its open days, Union Square is a pretty good market to rely on.
