You’ll get 60+ live TV channels you’re used to seeing like local FOX and CBS alongside ESPN, A&E, History and National Geographic. The subscription also gets you unlimited access to Hulu’s massive on-demand library and Hulu Originals that many folks pay $7.99 per month for. Before signing up for a week-long free trial, look over Hulu’s Live TV channel lineup to see if it’s a good fit for you.
Also, a little note, because my family uses ATT Unlimited for their cell phone service. I only pay $10/month for DirecTV Now. And every other package that I may want to try is $25 less than the general public (e.g. the "Just Right" package for me would cost $25, vs. $50 for the general public; the "Go Big" package is $35 for me, $60 for most others). Also, the premium channel HBO only costs me and anyone else just $5 more per month. I haven't experienced the "inconsistent video quality [and] a suboptimal interface" that you describe, but then again, I have 1 Gbit speed for my Internet connection. Even if I didn't have that speed, I think for the price of $15/month with live TV, and HBO to boot, it's a great deal, and DTV Now is getting better month-by-month. Yes, DVR is in beta, but it is going to happen.
The question of "when does it make sense?" can easily be rephrased as "how much money can I save?" and one of the biggest factors will be the price difference between combined internet and TV service or just internet. These are also the options that vary the most widely based on where you live, and the availability of promotions or contracts that can keep prices down for a short time.