I used to use DEXes like the Waves DEX and Etherdelta back when I was bounty hunting and trading altcoins actively and some coins were only listed on either Etherdelta or other smaller decentralized exchanges, but I also encountered low liquidity and thin books on most of the ones I did use back then, and I suspect those issues still remain on decentralized exchanges since I don't see much discussion on them at all anywhere.

I suspect DEX usage will probably become more commonplace as governments tighten regulations on cryptocurrencies and potentially start issuing more strict KYC requirements to exchanges, but they'll probably remain low-key for now.