Just a few cents from me:
I prefer wiki for the easier to use search functions and the option to edit relevant entries myself, as more experimental data is obtained during my playthroughs.
Online/offline is not a problem, there would have to be a nuclear war for this to start being an issue.
Ads don't really bother me, they are small enough that they don't distract me from reading.

What wiki lacks are mentioned pages with all items wearable, all artifacts, all monsters, etc. and the parameters of all these.
That's the part I like about IGB. Although wiki has discussions, I liked the way it was done in IGB.
A good example of that was the herbs page, which listed different ways of managing the issue and people could try any of them to decide what suits their particular needs/scenario/whatever.
I believe these would be the strengths that would make IGB valuable, since otherwise it can't really compete with the wiki.

Finally, IGB would have to be continuously updated because new stuff appears in the game quite often these days.
I seem to recall IGB being updated rather irregularly, which quickly puts it out of touch with what the game plays like at the moment. That doesn't happen to wiki.