I went to Oedo Onsen Monogatari in July last year and enjoyed it -- mostly. The only bad thing was dealing with the Yurikamome, which is slow as molasses, and in the summer has pitifully bad air conditioning.

They had bus service from near the Yaesu gate of Tokyo station, and Kachidoki, so those might be good alternatives (or take the Rinkai line to get closer and then hop on the Yurikamome)

I prefer the Heiwajima Onsen @ Big Fun Heiwajima more if you want to relax -- it feels a lot slower and kinda like your neighborhood sento. OOM is a lot more lively, and you'll find yourself wanting to switch between the baths almost every 5 minutes since the variety is quite large (maybe a bit too large).