Technically, under the hood beos/haiku is very awesome, but the look and the gui is so but horrible and ugly. If they made it skinnable and allowed you to modify the interface alot, I think it would be alot more usable. Its so horrid ugly. If the gui looked like/worked like amiga os, I'd use it I think.
The multitasking/multi-threading is better than anything out there. Its smooth and fast and effecient...
BeOS even came with an Amiga theme if you knew how to enable it. That's not in Haiku, but it could be easily recreated like the four it currently includes. (Windows, OSX, BeOS and the standard Haiku theme)
If this project happens I will tweak or replace a lot of things in the Haiku desktop (called Tracker) but not break compatibility with Haiku apps. No sense not taking advantage of existing BeOS/Haiku apps and development.
I'm doing a huge rework of UAE right now while continuing to look at the feasibility and how much I can get away with being native. Janus-UAE, although a very cool trick, isn't what I'm planning. This would be a lot more like Wine or maybe .Net since it needs a VM for the CPU.
Again, don't expect miracles, this will take a long time if it happens.