I have two omnibus's: Fantastic Four Vol. 2 (Lee/Kirby) and Silver Surfer Vol.1 (Lee/Buscema). They're beautifully done, but in practice, they're hell to actually read given the weight. Ideally you'd need a lectern to read them in comfort. If you want the classic stuff in collected form, the current Epic line and Marvel Masterworks is the way to go.

That said, if Marvel is committed to completing an entire run in omnibus format, I think they're worth the cost. Something like the Walt Simonson Thor omnibus' that collected the entire run is well worth it. But I'd only buy the best of the best in omnibus format. Some of the modern stuff that has been collected is a hard pass for me in terms of the omnibus.