AT&T
In the first couple of days, the answer has been poorly, for the most part.
CNET noted that DirecTV Now's Twitter account was dealing with a bunch of complaints on launch day, and that its forum was racking up posts (3,000-plus and counting). AT&T said it was just a launch-day problem, and that engineers had resolved the problem by day two.
But multiple Business Insider staffers, using both Apple TV and Fire TV devices, have continued to see channels stall and drop repeatedly. The bugs have resulted in a pretty awful time watching TV, though none has had the same issues on mobile.
Not ready for primetime
If you've never used a live-TV streaming product, this might surprise you. From a technical standpoint, Netflix is fine, Amazon is fine, Hulu is fine - so you might be forgiven for thinking DirecTV would be fine as well. But live streaming TV products have often been plagued with technical failures.
DirecTV Now's primary rival right now is Dish's Sling TV, which also promises your favorite cable or satellite channels delivered over the internet, wherever you are. This summer, after months of testing Sling TV, I wrote that while I had a largely positive experience with it, the product suffered from repeated technical failures, at the exact moments I really didn't want it to. Channels would get intolerably fuzzy or cease to work, particularly at high-traffic moments like the NBA playoffs or the "Game of Thrones" premiere.
Sling TV was a bargain. I loved the amount of channels I got for the price, and I feel similarly about DirecTV Now. (I even constructed the ideal cord-cutter bundle for myself at around $63 per month using DirecTV Now). But if AT&T doesn't nail the technical side, I simply cannot recommend it as a complete substitute for live TV. You want your streaming TV service to work every time you turn on your TV - like cable, or even Netflix - period.
Neither Sling TV nor DirecTV Now has showed me that yet. And it's not too much to ask.