re: Louisiana Purchase Cypress Legacy trees - Anybody seen one?

Pretty accurate when someone knows what they're doing. Obviously your error will get larger the more rings, but it is probably accurate to 1% in skilled hands. If some yahoo just cuts a tree down and doesn't know what he's looking at, he'll get it wrong 90% of the time. You can't just look at them and start counting. There could be years with extremely slow growth and the rings are close. Baldcypress usually has false rings also that are caused by short term weather events (a couple of weeks to a month), and not growing seasons.

It doesn't hurt the tree at all.

The only thing that is a PITA is labor involved. It only takes one or two people, but it isn't easy. Cost is free if you have a $100 core sampler and a powerful microscope