This is a useful portable instrument to have an approximate reading of radiation levels. It’s not for scientific purposes but it’s enough for safety. If the readings are too strong, just run away from that area. Basically with this instrument you can avoid radioactive hot spots.
This instrument is not meant to measure food or water, as explained in their web site: http://www.st-c.co.jp/air-counter/index.html but you can use it to have an idea of the radiation level on that area.
Personally I see this useful only in areas close to the delimitation zone, but quite pointless in Tokyo or other places with no danger. Basically because big cities are constantly monitored by many national and independent organizations with much more precise instrumentation. Anyway this is not a toy. If you are in an area with a potential radiation risk, this product could be very handy. I got it in Yodobashi camera for 6000 yen. Unfortunately the design looks like a pregnancy test machine or something related to vaginal hygiene… So you could look a little hentai using this on the street (^^)
Fortunately I got this before traveling to Europe, so I can get radiation measures in several places around the old continent. Currently Vienna seems to be close to Tokyo levels or even higher, but I need to take more measurements, let’s see :)