Ours are at eye level and we like 'em just fine.

I'm sure this is too late for you but I'm so glad you brought up the question. I will also need to decide and the info at this link shows me that there is not a simple "one size fits all" answer.
Let us know how yours turns out.
http://www.wikihow.com/Determine-The-Proper-Height-For-A-Pendant-Lamp
(I'm sure there are other places on the web to find answer as well but this looked pretty good to me)

Great article linked to above!

We ended up with the pendants being 36" off the island. they're large upside down bell shaped from Pottery Barn. The electricians have to come back to do some more things next week, so if I want them changed they can do that. Thanks for the article!

I think the better question is how far from the floor. I have a pet peeve against pendant lights hung so low that you have to be sure to look between them if trying to have a conversation across a counter. And they should be high enough that a tall guest (such as 6'5") can easily see beneath them if standing near them. I have the same comment for range hood heights. My tall husband gets frustrated to duck to cook with some range hoods.Or, to bump his head if he forgets to avoid hitting it.
lafdr

Something to consider--

In our lower level kitchen, the peninsula is 38" deep, and the pendants are 26" from the counter. My daughter accidentally hit and broke one of the pendant globes.

In the main kitchen, the peninsula is 44" deep and the pendants are 31" from the counter. A number of times, when drying large pans or cookie sheets, I've almost hit one of these globes.

Anne