I'm typing this from the new seed. First, Leopard is gorgeous. Not sure why you would think otherwise, but the UI is much more attractive and consistent.
I had mixed feelings about the transparent menu bar in the last two seeds, but the slight tweaks in this seed make all the difference. It's more opaque and blurs the underlying image more, so it looks good against everything. I've got my background set to change randomly every minute, with about a hundred different images to choose from, and they all look nice with it. You'll like it.
Second, the UI changes are such a tiny thing compared to the rest of Leopard that it's amusing to see so much time spent discussing it. Leopard is not about the new UI. It's got hundreds upon hundreds of small improvements that individually may not sound all that exciting, but all together make for a huge leap forward.
After using Leopard, going back to Tiger is like fingernails on a chalkboard. And I loved Tiger.
I don't want to break my NDA in a big way, so I'll just add one more thing. Leopard is fast. Impossibly fast compared to Tiger, and it makes Vista seem like it is running in slow motion. Spotlight is really instant now, to the point that I uninstalled QuickSilver. If you've used QuickSilver, hopefully you understand what a big deal that is.
For reference, my Leopard machine is a first-gen MacBook -- the lowest configuration, but bumped to 1GB RAM. My Tiger machine is a 2.66GHz Mac Pro with 3GB RAM. And the MacBook is snappier.
Wow, this sounds really promising. Leopard keeps getting better with every build!