I work very closely with numerous crypto exchanges for a living (I write code which interfaces with them). Outside of work, I've personally chosen to open a Coinbase account and trade on GDAX.
Coinbase is, in my opinion, the most reputable exchange out there by far.
It's a form of price discrimination. Only ignorant end-consumers pay the crazy broker fees. Anyone slightly sophisticated deposits USD via ACH (nearly free) and then transfers to GDAX and does their trading there (0-0.25%).
So you don't trade on Coinbase yourself?
Edit: It's almost they have one exchange for ppl who know what they're doing, & one to fleece noobs (3.9% on BTC/USD. I think).
As in, UX, UI, Docs, APIs, service design etc. are all top notch. Which would make it probable (but not certain) that the more technical parts are well engineered too.
Coinbase is doing perhaps something similar to what Stripe did for its field of business.
It was a real problem.