Speech recognition: it's a joke, isn't it?
As long as you make some noises in a number equal to the words to be pronunced and evenly spaced, the speech recognition marks as correct anything you would say! I managed to get a "pass" on saying something like: "hhh uuh eee" instead of Du bist Toll. So, it's a good way to exercise pronunciation, but don't expect that as a feedback of the quality of what you say!
At first I turned it off, but then I realized that it actually has some benefits. Without it, I don't practice speaking the words out loud very often, and my pronunciation deteriorates. With the mic turned on, I practice a lot more; and although it doesn't tell me when I do it wrong, just trying to speak the words gradually improves my pronunciation.
That's very true, more could be done however, considering I'm learning for free and it is encouraging me to speak, no major harm done. I can also play the recording back and compare to the speaker. =P
I wondered why the response was so quick. If there were a check involved that would require data being analysed a bit further and sent, there would be a wait time of about 3 or 4 more seconds. That doesn't sound like much but it's noticeable when compared to a 1 second feedback.
My advice is don't rely on Duolingo to replace a German tutor. It's a good grasp of the basics for reading and understanding but it isn't perfect =P