Of what will soon be all 300 issues, better than 2/3rds of it was reprinted in TPB form, maybe more if you count the spin-off minis such as Lady Constantine, Papa Midnight, Chas and City of Demons that have also been collected. The only long run that I don't think was collected at all were the issues in the mid-1990s written by Paul Jenkins, which I find somewhat understandable because it's my least favorite take on the character and almost caused me to drop the book for good. But it's also something of a shame because that stretch also had same really nice Sean Phillips art.
I'm not sure how well the trades have sold in comparison to Fables or 100 Bullets, but I'm also not sure if that's the fairest comparison, either. Both of those series can be viewed as "one giant story" with one main writer at the helm.