I was wildly enthusiastic about it as a DM and reader. Huge sandbox! Underdark! Non-linear storyline! Demon lords! Interesting and whimsical NPCs! It seemed a huge improvement after grinding through endless dungeons in Princes of the Apocalypse. The first few sessions bore out my enthusiasm. The players were awestruck and enchanted by many of the encounters. You know it's going well when they risk their lives to try and save a gelatinous cube...
...but it just fell apart around the breakpoint (7th level). It's, to date, the only official 5e WotC campaign that we simply gave up halfway through. It was a learning experience for me, and the key learning was this: sandboxes suck (for me, and my group... your mileage may vary). Anybody can do a sandbox. I've been DM'ing for more than 30 years, and I can throw random encounters at players without a book giving me a bunch of tables and small set-pieces. What I can't do (easily) is make up a well-designed dungeon on the fly, with unique puzzles and traps and riddles, with well-developed NPCs, and a compelling overall plot.
The first half of OotA has some very nice encounter areas that can be strung together to form a kind of plot, although it's pretty simplistic (escape from the Underdark!). The second half is, in my opinion, considerably less compelling. It relies a lot more on random encounter tables, and simply doesn't have the meat to support a full-featured 8th-to-15th level campaign. There needs to be more going on than wandering around tunnels for 10-20 gaming sessions, beating down the occasional monster, finding the McGuffins, and taking out the boss monster.
...would I recommend it to a new DM, knowing nothing about your style or group? Maybe. I think it's a good 1st-7th level campaign (maybe 1st-10th level, tops). There's enough content there for those levels, before getting very flimsy at the top end. There are many excellent and evocative encounter areas to explore. It's very useful having the stats for the demon lords, because 5e suffers from a lack of top-end opponents at this stage. Many of the NPCs are quite novel and interesting, although only a few have a real storyline. It can provide an interesting shock to players to find that the opening session could have their unarmed 1st level characters fighting AC 17, 90 HP elite drow warriors if they try and just slug it out... or their 4th level PCs could be squashed by a demon prince if they get cocky.
But be prepared to do a bit of heavy lifting with the narrative as a DM. You may need to inject your own storylines to punch things up, and the module is *really* lacking in detail and content beyond (say) the 8th-10th level mark.