To the extent you trust tether. But if trust is centralized, why use a block chain? If tether operated a Chaumian cash server instead of some block chain thing it would be universally better for all involved with no loss of trust from what actually existed before.
"stable".
Tether is always 1 USD so it is stable in the sense that the value doesn't change by 20% in a day which is what OP wants I think?
you can't redeem or use USDT for anything, and it's supposedly backed up by real usd, but in the terms of service, it says they have no obligation to redeem it.