I'm wrestling with similar questions and found the following opinion piece fairly informative, at least to give me an idea of the specific complaints regarding this crossover event:
http://www.hoodedutilitarian.com/201...ht-of-the-90s/
Hope this helps. I'm currently on the fence about picking up the omni due to the poor reputation of the storyline. However, I am leaning towards getting it - not so much for entertainment value, but more for the monument it represents - the moment when Marvel went completely off the rails, pimped out some of its most beloved characters to Image artists, and finally abandoned any pretense of preserving decades of established continuity. I am admittedly morbidly curious to watch the train wreck unfold. I missed this event back in '96, having become sufficiently disgusted with gimmicky covers, guest stars, artists defecting to Image, endless price increases, crossover fatigue, and editorial interference that I walked away from my favorite hobby entirely. Rediscovering my love for the medium through collected editions has been a true joy in recent years.
Plus, the completionist in me wants to see where Lobdell ultimately took his storyline with these characters. I've been reconstructing the Claremont run in collected editions and ended up grabbing most everything up through Age of Apocalypse as my nostalgic enthusiasm had its way with me. Operation Zero Tolerance (the only remaining follow-up collected edition of the era) holds no appeal for me, but it seems Onslaught would serve as the definitive jumping-off point.