I feel I have to agree with projectbook24mm that Helck had, "a very strong start and middle but a very weak ending," but I am going to have to disagree that reading it was a waste of time. The most compelling part of Helck is his extended backstory flashback -- and it is very compelling, really gut-wrenching in places. Once the manga moves beyond this point, and the battles get increasingly bombastic and neigh-apocalyptic, as each fight tries to top the one before it, Helck kind of turns into a grind. Also, the plot twists of the last few volumes, really start piling on to the point where, by the last volume, I hit the "WTF" wall, got pulled out of the story, and the concluding chapters lost most of their emotional impact. The final epilogue chapter just screams mangaka fatigue, and I would have been happier if it had been left out of the manga and all those explained plot points had been left unsaid and up in the air.
In my opinion, Helck passes the two-thirds good rule for being worth my time. The parts that are good are very good, and even compelling in places -- the good two-thirds outweigh the parts of Helck that are weak and not so good. Also, It is so rare for a 100+ chapter manga to conclude with a strong ending, or even with an ending that isn't a flaming train wreck, that before buying into reading a long manga like Helck, you have to be prepared to shrug your shoulders if the manga you like you gets a less than satisfying end.
I liked Helck. So, shoulders shrugged, time to move on to the next manga.