@ LoJack - Just un-installed Python completely, un-installed the 291 installer, manually installed the beta (kept my settings in place)
Non-CBash run :-
I had previously left Wrye Bash on the Installers Tab. Upon loading the exe it did a refresh, install order and install state of installers was all correct, went to mods tab, ran BOSS, I have one un-recognised which was placed last as it should have, Drag and Drop on that esp to a suitable position works, mark mergeable works.
Bashed Patch from previous build is 8,706kb, a new rebuild seems fine, all imports were as expected (plus the new csv's), after re-build Bashed Patch exactly matched previous at 8,706kb.
Load Game, No problems.

Not extensive testing but I am impressed. Still got to try C-Bash but for anyone else wary of trying it - I would say you are in for a pleasant surprise if you take the plunge.
Edit: Just about to try un-installing a few BAINs and re-install, including OBSE plugins, and see if I can make anything come undone, INI Tweaks aswell .. I noticed you also incorporated the re-sizeable separators on the Installers tab, tried and proved.
Edit2: - Found one Whoopsie, no Wizard install method available for those BAINs which have them.
BAINs now without a wizard option but manually selecting the sub-packages still installed okay though.
Otherwise, un-install and re-install seems to be working fine, I have a bunch of INI Tweaks in a BAIN which un-installed and de-registered from the INI Edits tab, then after re-install they came up again on the INI Edits tab, and work okay.
Speaking of the INI Edits tab - Request for any future version of Wrye Bash, can we have the column separator sticky its position http://www.bild.me/bild.php?file=8589414ScreenShot247.jpg
The column separator always reverts to a default spacing, this is the case with any current Wrye Bash version and not specific to the Beta (I just keep forgetting to mention it)
Edit3: The Screenshots Tab is fine, deleting old ones etc no problems - The PM Archive and People Tabs.. I have never used them so no clue if they are functioning as they should.