and my question is it more effective to simply hold a body-L until failure or should I be bringing my legs up until I am steady, then lowering down and repeating?

Barbell squats may be effective for getting a cut six pack

It's an entire kinetic chain in action, not just the front part of your core.

The people who swear by squats generally develop stronger diaphragms, a stronger core, and improve balance and stability.

This is why I love RC.com...

have had success with anything in particular?

I ask a question, sixteen different opinions that circumvent the question.

anyone got any suggestions for a core training plan or have had success with anything in particular?

L-
Awesome question. I hope that a few others chime in since, like you said opinions will vary.
Just to give a little bit of perspective on where I'm coming from w/ my answer... I'm operating under the assumption that climbers training their core are doing so in the hopes that it will lead to performance gains and are not necessarily concerned w/ injury prevention/rehabilitation.

So here goes: I think that the most needed aspect of core strength is shoulder stability. I know that this sounds weird but hear me out. From what I see, climbers usually have plenty of strength in the abdominal muscles and spinal erectors but lack strength in the shoulder girdle.
When I say shoulder girdle I'm talking mostly about the Trapezius(mostly lower), Rhomboids, Serratus Anterior, Latissimus Dorsi, and the Pec Major and Minor. These muscles all are involved in scapular stabilization to varying degrees.

I include the scapular portion of the shoulder girdle (versus the Glenohumeral joint) into the realm of core strength becuase mechanically it functions more like the rest of the core during climbing rather than as a primary force generator to move the climber to the next hold.
So basically what I'm saying here is that just as your hand is responsible for attaching you to the climbing hold so that you can manipulate your body around it; the scapula with all it's associated stabilizing musculature is what attaches your arm to your body. If this linkage is weak then it becomes the limiting factor in how much force you can apply from your body through your arm to move around the climbing hold.
Usually a lack of this type of strength isn't noticeable when the holds are hard to hang onto because your grip strength becomes the limiting factor. However when climbing steep terrain on big holds scapular stabilizing strength can very easily become the limiting factor, classic examples of moves that exploit this weakness are long cross through moves (especially when climbing in a face on/open body position moving off of a gaston), standing up into underclings and using flagging body positions.

How to train these muscles:
Ultimately climbing is going to be the best way to strengthen this area. This may sound overly simplistic but simply being aware of how you're using your scapulae during climbing is what it comes down to. When I'm dealing w/ patients who have shoulder injuries we talk a lot about scapular motion. The thing I consistently say is that unless your outside and are really psyched on sending something you should never hang on your shoulders.

Sure this takes much more energy and you may fatigue sooner and not be able to climb as hard but realistically if you're climbing near your limit your going to be engaging these muscles anyway to accomplish the moves on the route/problem. So by climbing with your scapula pulled down away from your ears you'll build more endurance in these muscles.
Perhaps most importantly when hanging relaxed on the shoulder the Glenohumeral joint is put in a position of impingment, the ball of the humerus is not centered in the joint and therefore any motion of the joint causes friction between structures which can lead to a variety of shoulder problems from bursitis and tendonitis to osteoarthritis.

I definitely like using free weights to train the scapular stabilizers since free weights present a much more controlled and quantifiable environment than climbing. The primary exercises that I use to strengthen this area are:
One Arm Rows,
One Arm Overhead Press while standing on the opposite leg,
Dumb bell Bench Press (one side at a time)
Supine Tricep Press w/ Dumb bell
I do everything unilaterally becuase that allows me to really focus on how my scapula is behaving when under load. The scapula should never be forced to any extreme of it's range of motion, the muscles surround the scapula should act as shock absorbers to maintain the scapula in a "neutral" position. The easiest way to find neutral is to stand up nice and tall and then simply turn your hands so that your palms face forward. You should feel you scapulae drop a little and move slightly toward your spine, this is usually accompanied by an feeling of opening through the chest.
That's neutral and is where your shoulders should be when lifting and climbing. I realize that you won't be able to keep your shoulders there all the time when climbing but keeping them there as much as possible will help keep your shoulders healthy and also help develope more "core" strength.

Well that's my take on where most climbers are in need of core strength but that may just be the population of people I see and be irrelevant in a lot of situations. Hopefully my thoughts were somewhat coherent. Please get in touch w/ me if you've got a question or a point you'd like clarified.
cheers,
BA