i have a thudbuster LT on my Flash 29er. The only issue I have with it is some clunking on rebound when the elastomers get worn. Could be lighter, I suppose. Other than that, its an awesome addition to a hardtail. I couldn't imagine riding a hardtail at my age (49) and having back issues without a suspension seatpost.
Came by a ST Thudbuster unexpectedly. Tried it, liked it a lot - now part of my HT ride. Also have a USE-SX suspension post on my CX ride. Feels sort of mechanical - I much prefer the Thud as its smooth and almost non noticeable. Your current post is probably stamped with diameter - that's what you'll need to order new.
Skip the Thudbuster and get the best for comfort, a NitroPro gas suspension seatpost. Definitely not cheap but way comfy and it doesn't buck you off like a thuddy that is not damped at all. It uses a nitrogen charged cylinder for the air spring and a built in oil dampened shock so after absorbing a bump the seatpost rises smoothly and controlled so to not buck you off the seat. The dampened seatpost also does not bob up and down when you are pedaling hard.
Definitely made my harsh riding Caffeine frame more enjoyable to ride using the NitroPro. It is durable too as I have used mine for over 1,000 miles with no problems and I am a big guy at 6'4" 250 lbs. There is a built in wedge adjustment piece that can be tightened if any rotational play develops over the years.