It's been discussed before that the reason most people don't read or post to Meta is that it is way down at the bottom of the page. The Off Topic forum is almost as far down (only 5 above Meta). It's likely to be ignored almost as much. I predict that the mods are going to end up moving alot more threads, and no matter how many times you tell people "It belongs in Off Topic" threads will start in General, only to die in Off Topic, except for those of us who will spend a good amount of time there. Breaking the General forum in half (which is essentially what this is doing) will just result in less people regularly posting in both areas. Some will migrate to Off Topic and spend most of their time there, others will stick with General, depending on what they prefer. The amount of discussion in each will not be the same as we had in General alone.
So, my vote is not to have the Off Topic forum. I like it all in one place, where discussions can attract people who wouldn't see them otherwise.