OK, so I've now spent a day or so playing with the 2.0 beta.
The new Write page is great. The collapsible side bars are a really nice touch, as is being able to add post categories while writing a post. The Image browser is a nice addition too, in theory…
Unless it's my imagination, everything is slightly quicker. Also most of the plugins that I use still work.
The stability of the javascript image browser and text editor on the Write page leave a bit to be desired though. I've already written this post once, but Mozilla died horribly and I lost everything. Earlier, I was playing with the image uploader, and managed to kill Mozilla again too. I've now turned off the rich text editor and I'll see how things go.
I think I've ironed out all the issues the 2.0 upgrade caused – my comments stopped working properly, as did search. Search still isn't fixed, but I'll do some more analysis later. Also, using the image uploader caused some nasty CSS issues, so I'll need to fix that if I'm going to start using images a little more
Other than that, I'm very impressed with 2.0. I think I'll stick with the beta for now; I can't see any real reason to downgrade to 1.5.2 again.
If you are going to try the beta, I'd suggest BACKING EVERYTHING UP FIRST! I can't stress this enough. Breaking your site whilst trying out the beta would be A Bad Thing, and you'll only have yourself to blame if you can't revert back to a working version. Back the database up, and back your files up. Twice 
