I agree about H/C - core strength gains occur almost overnight. The DVD's are long (45 minutes - including warm up). You can break them into segments but there is no chaptering. They are worth getting. I have yet to finish bootcamp. I collapse each time - literally, around the 43 minute mark.
For shorter, effective ab workouts - I agree about Ab circuits. There is great variety on that dvd. Butts & Guts - there's a great, long ab workout on that.
Look into Exhale Core Fusion workouts. Arms + Abs is good. There are two ab segments - yikeserama. Their Pilates Plus is equally challenging. I have other ones too. They are short but boy - do they pack a punch. Instant skinny.
Anni's Fierce Force - her core section is tough too - it's a mix of cardio and ab work.
S90 - Core Ball - that's a good one too. It's about 30 minutes .
P90X - Abs Ripper is also tough tough tough.
The Squeeze's have surprisingly good core work too.
These are my top go to workouts for core strength. I hope that helps a bit.