I've used sample code parallax provides for SHT21 and it appears to be working - humidity readings do increase when I breath on it, or decrease when I put it to dry situation. The problem is that, I have accurite indoor humidity meter, and it's reading, which I found pretty close to real ones (according to personal feelings), are completely different. When SHT21 shows 45%, accurite shows 31% and so on.
As far as I know, that SHT21 has "true" I2C, wheile SHT11 - does not. I've tried to use both BS2 and BS2P boards - no difference.