I have a Ti post that I haven't used yet (it's on deck, so to speak). I think there are always siezing issues with Ti stuff, which is why they make anti-sieze. Grease works too, I understand, but you may be tempted to use more of it than you should -- after all, you don't want that seatpost slipping around.
All in all though, don't worry about it if you're using the right stuff. Companies like Lightspeed and Moots have paired Ti posts with Ti frames for years.
There's no super compelling reason to select a Ti post over a carbon or alloy one, except that Ti is an interesting material. If you're considering one, that means you're in love with the idea on some level. Go for it if the price is right, says I!