For the most part, I really like adv/disadv, and that's mainly because it's a reasonable simplifier, is not granular and does not provide a like benefit across the full range of possible rolls. For a game attempting to be truly "realistic," that sort of specificity might matter. But D&D is a story-generator, not a realism simulator. And if you think about "advantage" and "disadvatage" as those words and not numerical modifiers - ie, as a nebulous and fictitious "something" that might impact the outcome of a fictitious event - then the mechanic achieves exactly what it sets out to.
The main complaint I've got, as others have also suggested, is that it doesn't "stack". But in the vast majority of cases, it seems fine. A couple disadvantages versus one advantage doesn't strain my suspection of disbelief too terribly much; and in the rare event that you see 20 disadvantages versus 1 advatntage, it's easy enough to make a ruling to accoutn for that and move one.