I found Berserk utterly compelling for the story arcs that dealt with Guts' coming-of-age with the original Band of the Hawks, and the spectacular rise and fall of Griffith (basically, the storylines that appeared in the anime series). However, the manga started to run out of steam when Griffith disappeared from the storyline for far too long.
I think it would have been a much tighter story if the author had stuck with Guts, Griffith, and the original Band of the Hawk--for one thing, we'd spent a long time getting to know Pippin, Judeau, Rickett, and the others, and while killing off everyone but Caska and Guts was a truly shocking turning point, it also meant that the author had to stop the forward momentum while while Guts wandered around and gathered up a new set of companions. IMHO, this really disrupted the story's pacing...it feels like the author is doing the same thing twice.
However, now that the New, Improved Griffith has reappeared in this interminable Millennium Falcon arc, I'm finding myself more interested to see what's going to happen.