It depends. I'll often read the first book in a trilogy and find myself getting distracted by other books and not jumping to the 2nd and 3rd anytime soon. When I'm a year or two away from the omnibus release, I often don't bother buying and reading the rest and take my information on the plots from places like these or talking to others. Its basically the same as with the Gathering Storm right now - I didn't read them and am not sure I am inclined to do it, so I ask others or follow discussions of the contents. That way I can stay in the loop while being relatively patient.
But I like owning omnibus editions with their extras. I still haven't finished Ahriman; I read Exile and picked up Sorcerer, but didn't get around to Unchanged because it fell into the post-AoS and marketing brochure timeframe and I couldn't give enough of a damn. Getting to pick up a complete Ahriman Omnibus including all three books plus the short stories means that I can move two trade paperbacks off my shelf, replace them with a slightly thicker omnibus, and have the complete package. Same goes for Legacy of Caliban - I own all three trades, but having the extras in an omnibus and saving space through it is great, and since I enjoyed the books, I don't mind buying a combined edition.
I haven't read the Deathwatch novels, but a few of the short stories that'll make its way to the omnibus, so that's a no brainer at the pricetag.Angel of Fire I picked up in hardback for half the supposed price due to a price error at the retailer, but never felt inclined to buy the rest in the same format, so I was always going to wait for the omnibus, and they took their sweet time with that.
It often is very dependent on extra content whether I bother with the omnibus, but they're great value propositions, especially when I have yet to buy and read the third book in a trilogy.
I'm very happy they're reprinting the Time of Legends omnibuses (though I fear the format will change from the large HH-sized trades to regular omnibuses), because recommending those while they were up for 50-100 bucks on Amazon or ebay was a sodding pain. I bought all ToL up to the format switch in MMPBs, and the three omnibuses (and they're beautiful!). Malekith I actually got twice in MMPB because my original copy got battered in my bag a bit too much, and I own Horus Rising in three formats (though I don't intend to get the omnibus HHs).
There's a bunch of classic omnibuses they've been reprinting lately, like the Word Bearers trilogy and Iron Warriors, or Ultramarines. I got the latter in their original editions, but only Storm of Iron in MMPB and none of the Word Bearers. The upcoming Sisters of Battle omnibus will also be replacing my old Faith and Fire. On that note, one of those days I'll have to take the plunge on the Night Lords omnibus. I don't know, the cover just doesn't excite me as much as Sullivan's art on the MMPBs...
Here's hoping there's actually going to be another print run of Gotrek & Felix's reissued omnibuses soon. I missed out on the run last time, and my battered or german copies just don't hold up next to the final books.