Apparently it's only loosely based on the comics, and the ending is different. Thing is, some reviewers I find I generally agree with have said it was decent enough. Not as good as Superman, but not terrible either. Reminiscent of 90s stand-alone superhero movies like Blade.
I tend to be sceptical of "bad writing" these days, because it's the standard generic complaint made by people who don't have genuine criticisms. I haven't seen it so can't comment on the look, but unfortunately a lot of modern movies do look quite bland and dull. The look was one of the things I liked about Master of the Universe, which pitched itself just right to be enjoyable, a lot like Gunn usually does.