Sure, it's HBO, and sure they have some stellar in-house programming; but it's one channel.
Not exactly - because it's not just "HBO Now", but also "HBO Then".
That is to say, you aren't just getting current seasons of stuff like Game of Thrones - you are getting all the seasons they aired. Plus things that aren't even airing that were great - like Rome. Plus many more somewhat-current movies than Netflix has (although to be fair Netflix has a small number of near-current movies I actually like to watch, whereas almost none of the movies HBO listed were interesting at all to me).
That said I may just subscribe during Game of Thrones, then cancel again... it'll be interesting to see how I feel at that point.