The idea behind Tether (USDT) is great, stablecoins serves betetr than traditional fiat. USDT is widely used (the most common used stablecoin in crpto space) and you can use it in the exchange for almost any crypto asset. On the other hand there is risky part of the USDT, the company behind this stablecoin never allowed to audit its fiat balance which is pegged to USDT. As more USDT are minted there could be an inflation which will shift real price of USDT much lower than USD if there will be no guarantee of same balance in fiat account of the company.
It has the third largest Market cap.