The performance envelope for the Zero-Zero seats is considerably bigger than their predecessors.
Here's a good link explaining the concept, and a test:
http://www.ejectionsite.com/project90.htmThe ejection envelope is dependent on many factors, honestly. Aircraft type, aircraft altitude, aircraft orientation, airspeed, you name it. The newer seats have come a long way, and can mitigate some of those factors, but aren't 100% and never will be.
What won't work? A super-duper "smart" ejection seat like the ACES-II isn't going to help you much if you're inverted over the deck by just a few feet, or if you have so much airspeed going that the opening shock of your 'chute breaks your neck/back.
However, it's definitely better than the alternative. There are some impressive videos of Zero-Zero seats being used to good effect, including that USAF Thunderbird pilot who punched out a fraction of a second before his F-16 pancaked into the deck from a poorly-executed outside loop.