Thanks for your points, +wasaweb.net
and +Mark Hurn
. Just to expand, it is clear that the problem comes in with idiomatic phrases. If you know enough of the source language you can usually spot them, and spot when a translation isn't right, but most of the machine translation I have seen just doesn't handle enough idioms, or handle them well enough.
Consequently, it works well for some kinds of documents - technical manuals, eg - and less well for others - blog posts, people's "talking." Journalistic pieces tend to fall in between. The more formal and uptight the publication, the better machine translation does.
(Amusingly, this post appeared in my stream just above a post in Spanish, which used an idiom that Google Translate did not handle correctly.)