Most tree harnesses take up more space than my entire summer backpacking kit which is a big deal when traveling via bike. But I must admit that if I had the money I would own a tree harness for day climbs.
Yes, I bought one a couple months ago. Love how light it is! Unfortunately the seat squeezed my legs together and crushed my hips. Couldn't sit in it for more than 30 seconds before it hurt. While I was climbing, the seat would creep up and ride above my buttocks, so I constantly had to pull it down before I put my weight into it. Had to return it. I would give it another try if it had leg loops instead.
+1 for Sequoia. Nothing else comes close to it for weight (that includes a rope bridge and side Ds). The Skylotec Record is the closest thing, by having a rockclimbing belay loop style main attachment, except instead of a dyneema sling you have a ring. Semi-floating bridge but not quite