When cleaving meat you need a lower surface. Not too many people actually use them for their intended purpose, so they started 'growing'

If you are using it just like the rest of the counters then the same height but if using for a baking center and you make your own bread from scratch a lower counter works better for kneading bread or rolling pastry.

Remodel is right, that said I would make it counter height just for common use purposes. I'm guessing that is more what you are going for anyway? Is it connected to the counter in any way or it's own free standing piece?

We have a butche block table and it's the same height as the counters and looks/works well, in my mind.

And 9 yrs later for those researching now, I will add that we went a bit higher than countertop height which ranges from 34" to 36." We had two islands and on the second I wanted end grain 3" butcher block. I love it and nothing else conpares when rolling out dough. We are taller and wanted a bit more height and went with 38" which is just tall enough to be distinctive. If you are building a custom kitchen customize it a bit. Some people are fixated on resale value but we hope to stay in this house long enough to make it worth making it ours. Still if we were selling today, the kitchen would sell this house.