There are several 'amateur' maps of radiation being put together, and the comments I listened to (BBC Radio 4 program - I forget the name) from professionals in this area, was that while its of some interest and use, the big problem is that these devices are not calibrated.
So, the readings are almost meaningless, apart from indicating dangerously high levels.
And as Mike says, unless you have a suitable thin-ended tube, the alpha won't get through the glass.
However, if I lived near Fukushima, its the first thing I would make.
Incidentally, I made mine from the charging circuit of a disposable camera, so the tube is the only bit that needs to cost money. Although, mine is clearly not as neat