While I don't think it was truly terrible it was still pretty weak in the end. They could've presented a much tighter and interesting story than the drawn out and semi bland one we got instead.
Not to go off fully on a tangent here but I feel this way toward a lot of modern movies (specifically blockbusters) that have a 2 hour+ run-time. They have more space to work with but they almost always tend to fill it with drawn out spectacle versus better written characters, more cohesive plots, and tighter pacing. I just find myself walking out of these films let down and underwhelmed. A lot of my favorite films from 20 years ago and prior just feel more substantial even with sometimes under 2 hour run-times.
Also don't get me wrong I love long films. The extended LOTR saga is a fine example. They made them longer and I loved them even more.
A perfect reverse example is The Hobbit Trilogy. Trimming the fat and sticking to the original story would've produced a much better product.
Coming back to BVS they could have easily made a tighter film and still included elements leading up to the Justice League or extended DC Universe. Whatever you want to go with. If you really boil it down there is so much bloat here that even though lots of events happen it almost feels like very little did. By the time it finally gets to the confrontation between Bats and Supes it just feels rushed and underwhelming and is then over before it even started.
Then it jumps ship and swerves right off from there into an entirely different scenario abandoning the original plot altogether.
Again although I don't think it's terrible it gives the impression of multiple film plot lines (BVS/Man Of Steel 2/Justice League Origins) ham-fisted into one lackluster film without any real satisfaction or payoff.
So all in all I disagree with those saying it's totally awful because there were few interesting and well done moments. Basically it was good enough for a one time experience. Just like Man Of Steel. Still fun is not a description I would even begin to apply to this film.
TL:DR Drawn out and kind of bloated but still watchable though fun it was not.