I installed build 10049 on my second hard drive. It has the same bug as 10041 where the Mail app does not install correctly. Everything else seems fine. I tried out Spartan and I liked it. It really is spartan right now since a lot of the settings are not present yet. Things like tracking protection lists, search providers, and most of the internet options. It does work quite well though on every site I tried. I still like the appearance of IE 11 better though so I'm not sure which I will use as my default when Win 10 launches. I installed Classic Shell 4.2 and disabled Cortana and the search box.
I really only want Win 10 now for the increased gaming performance that comes with DX 12 and the new drivers supporting it. Even in DX 11 games, I'm already seeing a 10-15% increase in frames per second. Windows 10 promises to be revolutionary for gamers.