I'd say it is worth less than a +2 to a stat. It counts as a double constitution bonus for hit points, however, it misses out on other things that might benefit from a con increase. You lose out on +1 to con saves or ability checks, barbarians miss out on +1 to AC. I think it's value increases when a PC already has a maxed out constitution as it boosts your hit points beyond what would otherwise be attainable but then if you already have a maxed out constitution then you probably aren't really worried about a low hit point total.
If bought instead of a primary ability increase then you also have to take into account what else you will gain from them. I do feel that 5e is fairly forgiving to players who don't max out their main attack attribute so that not gaining that +1 to hit/damage isn't as big a deal, but there are also all of the extra uses for that attribute that you need to consider (skills, ability checks, saves, initiative if you are dex-focused). I think that basically, unless you are really worried about low hit points or want to create a tanky character that can soak up a lot of damage that the durable feat is a feat that you pick up later in your career, if at all.