Orange is underwhelming not really because of bad adaptation or source material, but due to the hype it was riding on.
When the announcement and trailer of Orange came out, many are impressed: elements of time travel, romance, and tragedy is enough to garner a good following for the upcoming anime. In a sense, it supposedly share similarities with the anime that started the crazed hyped over time-travel, Boku dake ga Inai Machi, two seasons ago.
Here comes my pet peeve: supposed MCs look like clones of Kou and Futuba from Ao Haru Ride, the most successful romance-shoujo-"drama" yet. Also, Orange seems to share this same genre.
The thing about such a genre is that there's little variance to work with, and many works and adaptations out there simply became clones of each other. I can provide you a list of recent similarities but its very easy to search that up.
So here comes Orange, which promised us something new from the old and beaten genre, with an additional niche of time-travelling included. The end result of which is utter disappointment, for Orange fails to be out-of-the norm, and the niche is gimmicky.
Orange's Characters fall into the generic characters of this genre. The plotline is also generic, broken to down into mere phases of lovey-dovey and mistrust. Anyone hoping Orange would be different would be left disappointed.
Though, that is not to say there is no merit to Orange. For one, it is the best of its genre in my opinion, beating Ao Haru Ride in terms of enjoyment, with the time travel and impending fate of a certain character somewhat driving the slow plot progression.
With the announced extra episode, I hope many of the pitfalls to the generic Orange fell into can be remedied, one mainly is the development of the other characters. Unlike the other, I would say Orange's supporting cast is a huge untouched potential that could be (and should have BEEN) developed.
There is, in my opinion, two camps in viewing Orange. One, not having been exposed to this genre before, would enjoy Orange greatly. The other camp, realising what Orange is, becomes bored, and the only reason they watch the show was to see the conclusion (aka the time-travel gimmick).
Whether Orange is your cup of tea or not, one consensus we can share that at least visuals and audio is impressive enough to give it a shot.