I can't believe Hanoibloke thinks that this is the smallest market in Hanoi!
This is (currently) my local market and I have to say that its probably the best stocked market I've lived near in Hanoi. The only exception to that is the fruit issue mentioned by the previous reviewers. There are plenty of fruit sellers around outside the covered market though I'd recommend buying from the ladies with pannier baskets rather than the shops as they generally have better quality and cheaper goods.
Inside the market the vegetable sellers generally give good prices. I'd recommend sticking with one provider, this way you'll generally get decent prices (if your Vietnamese is up to it). If ever I can't find something I'll ask my helpful veg lady and she'll grab it from someone else for me. I've found lots of things here I've never seen in other markets such as (curly parsley was the latest nice surprise).
I do the same with the meat stall too which, although slightly more expensive than elsewhere, is good quality. You can have your meat minced, and they sell all sorts of useful cuts (for the adventurous cooks amongst you) such as tongue, trotters, hocks, solid fat etc. The normal cuts of pork and beef are good too.
Two legged meat (i.e. fowl) can be found in a separate section generally comes in 2 varieties: fatty chicken, and skinny, Ga Ta.
The seafood section is also very good, there are usually plenty of live large prawns, crabs, and fish; and a few of the vendors sell squid too.
I've been told that some of the vendors here buy their produce from farmers directly and not from the big wholesale market in Long Bien, which is another plus if true.
The only thing that I don't like about this market is that too many people drive their bikes around it, but that's hardly peculiar to this market.