Shinagawa is a bit boring seen from a touristic perspective. Though highly convenient for the trains. Look in to the Ueno area (Keisei Skyliner), Ginza/Marunouchi areas (N'Ex to Tokyo Station). There is a lot to see and do there. If you want to go to Shinjuku or Shibuya for sightseeing and a night out, I would probably still recommended a hotel on the Tokyo to Shinagawa stretch for easier access in the morning.

If you look in the Japan forum, this topic is frequently discussed, and there are lots of tips and ideas.