Yes its a very useful document even though you can print it out yourself - has to be accompanied by a F1250 or F90 for the brown-jobs to make it valid.
As well as USA it gets you in to Turkey without paying the £10 for a visa at the entry port which normal EU peeps have to fork out for. I've also used one on BA from LHR to Frankfurt when my passport was away getting visa'd up for a different trip and for entry to Romania when arriving by Crab-Air.
Top tip for US trips (or visits to US bases in Germany et al) make sure you have a specific NTO for the duration of the trip and that it specifies the locations you're visiting. Blanket orders such as All Countries, All Borders and valid for a year (all perfectly kosher elsewhere) don't cut so much ice with The Cousins - all akin to US Mil Orders as has been mentioned.
