IDEX has been the #1 DAPP on Ethereum since March 2018[0]. It's also the #1 DAPP by users and transaction count.
You can't buy crypto with fiat there, though, so connecting your orthodox bank accounts is irrelevant in this case. The founder said yesterday in an interview [1] that they may work with partners to enable this eventually, but so far I don't know any good decentralized exchanges that offer that. Buying crypto on a decentralized exchange with your bank account seems pointless to me - either buy it in cash or buy it on a safe platform (and then get it out immediately so they can't exit scam you or get hacked with your money).
Bisq has that and has been around for years. Want a single, stable, smooth and fast fiat gateway with strong guarantees like you have on he realized exchanges? I think that's fundamentally incompatible with not having intermediaries. As others mentioned,calling IDEX decentralized is weasel-wording at best. It's a hybrid.
IDEX is semi-decentralized: order-matching is handled off-chain, funds must be deposited into a centralized smart contract that handles all settlement, but the smart-contract itself and balances within it are held on the Ethereum blockchain. It prevents some forms of exit-scam or hack (you don't need to trust that the IDEX principals won't send your funds to a bank in the Bahamas), but retains vulnerability to a bug in the IDEX smart contract allowing a hacker to obtain them. The IDEX smart contract is at least public and auditable, though.
There are 4 core components to a decentralized exchange:
(1) storage of funds, (2) order books, (3) order matching, (4) settlement. Unless you have all 4 of these it is a hybrid exchange. Calling it anything else is misleading an disingenuous.
You can't buy crypto with fiat there, though, so connecting your orthodox bank accounts is irrelevant in this case. The founder said yesterday in an interview [1] that they may work with partners to enable this eventually, but so far I don't know any good decentralized exchanges that offer that. Buying crypto on a decentralized exchange with your bank account seems pointless to me - either buy it in cash or buy it on a safe platform (and then get it out immediately so they can't exit scam you or get hacked with your money).
[0] https://twitter.com/idexio/status/969609728329469952
[1] https://www.youtube.com/watch?v=60rvPilAmDk