Although I fully agree with Pen_samurai about Itoya regarding service, the one thing that you simply cannot predict as a foreign visitor is what would be the available Nakaya on the day you visit.
I just returned from another visit to Tokyo last weekend.
The last time I went a year ago, I picked up a LOT of regular production Japanese pens from all three manufacturers, Sailor, Pilot and Platinum. I bought the majority of the pens at Itoya and they couldn't be nicer or more accommodating.
This time, I wanted to spend on just one nice pen, namely a Nakaya. I knew before going, based on information from the Nakaya website, that K. Itoya had the best selection. Imagine my disappointment when I arrived only to find they had a very meager selection. 3 Piccolos, one carbon version and a pink and light violet urushis, about 5-6 portables and longs, and 3 Chinkin pens. None of them were the designs I wanted. Many of them were fine and medium nibs with only one that's broad. Also I was told that even if I wanted to buy one of the existing pens, I had to wait a month for the pen to get shipped back to Nakaya and switch to a Music nib, which was the nib I wanted. I am guessing this is because stock rotation is extremely fast at Itoya, and it's almost impossible to predict what would be available. Also I am sure that Nakaya would send pens that are easy sellers, fitted with easy to sell nibs.
I ended up buying a European pen in Itoya instead that was impossible to find in the US. The experience was nothing short of amazing as described by Pen_Samurai. I initially had a bit of problem with skipping on the nib. The salesperson was extremely accommodating and had the nib adjusted right away by the pen tech. The pen tech even came over to look at how I used the pen, and suggested a slight change to how I held my pens. Which was a revelation and delight. Truly an experience you could only have in Japan with it's extremely customer centric service and great stationary retail culture.