The worst thing is feeling in the way—these guys are working, not here to entertain, despite their surprisingly quite accommodating friendliness. The second worst thing is being torn between drooling for sushi and being slightly repelled by the still flapping tails or snapping claws.
Getting in is slightly confusing—there isn't really a visitor's entrance and you have to go in the way the workers do and keep your eyes peeled for trucks and their extended vehicular families. On the main road, there are noodle bars and stands selling fresh, cooked fish of all kinds, but with just a bar and four chairs—they're not really the restaurants that the guide books promise, those are around the other side and in the narrow lanes beside the main undercover market where along with seafood you can buy crockery, dried foods, pickled foods and other unrecognisable things, suction packed in plastic, fruit and vegetables and anything else you might need if you were planning a dinner party or opening a restaurant. If you're in the market to buy something for your tea, get some advice from one of the less hurried (possibly older) looking market men (not one driving on of the mini trucks), and see what they recommend that day, the people were surprisingly friendly considering I wasn't even a proper shopper, I was taking photos and possibly in the way (though I was trying not to be.)
Nowhere else like it—a perfect place to discover the marketplace atmosphere of Japan and the culinary traditions all in one go. Definitely go, you won't regret it. Even though it's an early start to get there before 8 am.