You really need time to explore this place, it takes a while to get the hang of it. It's about 10 minutes walk from Porte de Clignancourt metro stop at the north end of Line 4.
I love the mix of the modern graffiti with the old objects.
The market runs along the Periferique so it's interesting city scape where you move away from the old style centre and find yourself under flyovers etc.
It's like a small town in itself. I tend to go on Mondays, maybe the prices are better then because Saturday and Sunday ( these are the 3 days it's open) are very full of tourists. I don't always haggle but it is the norm there - often I'm buying stuff from the really messy rows of boxes of junk and they've never charged much so I would never dream of haggling. It's a real feast for the eyes - you don't have to buy anything it's interesting just looking - though I find I can quite quickly be overwhelmed, there's so much to see.