The negatives are obvious; less sociable, more easily irritated, wanting to be by yourself. After you've spent a day in the zone, you're not really "party material".
The positive (apart from being very productive) is that I use it to get my negative feelings out of the way - anything that is bothering in my life is somehow gone when I am in "the zone" - it is truly a zen feeling as the author explains.
It's also important to mention that you can't force yourself to be in the zone. It comes and goes, with very little control on your behalf. People that try to force themselves in the zone by working harder, are not truly in the zone. It happens seamlessly without you even knowing or wanting it.
For instance, I'm hardly in the zone. It happens probably once every two weeks, if not less - it also depends on what I'm working on; if it's something new and exciting I'm more predisposed to get in the zone.
Being in the zone is like getting an adrenaline rush - you can force yourself to do it more often (go skydiving for instance), but if you do it too often you'll quickly drain out and not enjoy it as you used to.