I've lived in the area for a couple of years now, and decided in the Spring to try each of the sushi restaurants within walking distance -- the total number is in the double digits if you include the sub-par AYCE places in and around the mall, but I focused on the five main ones - Asakusa, Yakko, Yakko Express, Sushi Garden and Isami. Isami is easily the best.
My initial two or three visits to Isami were for takeout, and the food proved to be very good, with good presentation despite being, well, take out. On the basis of food alone, I figured it to be about the same as Sushi Garden -- very good. However, having had the opportunity to sit down at both restaurants, Isami is a MUCH nicer experience overall. The ambiance is wonderful, with fairly dark lighting and decor. The service was great, without the need to ask for tea refills, and an older waitress (who was definitely Japanese) who always bore a soft, unforced smile.
I was also delighted to find rolls that weren't on the take out menu...particularly the pelican roll, which is beef teriyaki and prawn tempura together. It was nothing short of delightful, and I found myself quite full after only three orders of rolls.
Ultimately, this is now my go-to spot when it comes to Japanese food, as it combines great food with friendly staff and welcoming decor.