I use online translators pretty regularly. They're good enough to get the general jist of what a piece of text means, and provide acceptable translations for basic communication in general conversation. Depending upon which languages you're translating (their complexity), the results are hit-or-miss. But for general-purpose use, they're decent.
But when looking to translate something more technical, they're not so good. I mean, look at all the Engrish / Chingrish manuals that we get. Sometimes, the translations are so bad that the manuals are useless. Using an online translator to convert English to Japanese would give a translation comparable to that.