The Hollow operates in the same pacing and logic (or lack of thereof) of a dream, things come and go quickly, the world around is a mystery and nothing really makes sense, and that might be a deal breaker for people that wanted a long and compelling story, rather than just a fun ride. The art style is a bit ugly, but it is in no way lazy and the animators do their best.
The biggest let down is definitely the ending, regardless of what you wanted it to be, or if you already knew early on, it is very hasty, and very very very poorly made, and it has one of the things I hate the most in animation, it just made me cringe and almost ruined the show. The entire time I was thinking "this is one of those shows that really starts at season 2", but after that ending, I'm not sure it will or if I care.
Still worth watching if you have free time, just don't bother if you find yourself not liking the characters, staying to find out the conclusion alone is not worth it.
Strong 6 to a light 7