I bought a batch of ESP8266-01 and DHT11 on ebay.
The sensors give data, but the datasheet say +/- 2 degrees. Who can live with that? Well, they claim the repeatability is 0.2 degrees, so I guess they could be calibrated.
But thet are not stable. They start a few degrees over room temperature, but gradually the reading is up to 33 degrees, more than 10 degrees too high.
Could it be the proximity of the ESP8266 giving off heat? Or are the sensors themselves heating up when reading at one read every two seconds?