Isn't it neat how a software vendor can force you to abandon a working product to migrate to a broken one. That's so cool! I'm in the wrong business.
Change can be challenging to manage and accept, but I don't agree that the product is broken. We had some issues with 11.0.5 but none that were show stoppers in terms of migration. We additionally migrated from CQM to DQM packages as part of the 10 to 11 migration and are none the worse for wear.
All our customers are on either 11.0.7 or 11.0.8 at the moment without any heartache. In fact, it is quite the opposite considering all the new functionality not available or possible in version 10: Mapbox mapping; data modules; file uploads; dashboards and stories; customizations and extensions; improved search and recents; etc.
As for the authoring interface changes, the main advantage is having a unified interface that accommodates all types of users. In my experience, new users with no experience authoring in version 10 pick this up much more quickly. Authors with experience on 10 sometimes have a small hurdle finding things in a different place but manage to adapt without tears or fits.
A recent webinar run by IBM on reporting techniques included a mention from the product manager that they plan to allow fly-out menus to be repositioned in a near future release. In the meantime, if it is in the way, just click a second time and it goes away. I also direct users to the RFE site so they can vote on features and enhancements they'd like to see. IBM actively monitor this site and take these into consideration in their release planning.
Of course everyone has different opinions and experiences so take mine FWIW. I would add that even though 10 is out of support in April that doesn't mean people are forced to upgrade. I wouldn't expect IBM to provide a patch to fix a bug that may surface in v10 but if it is working fine then I don't think it will suddenly stop working in the very near term. There are people still out there using Series 7 Cognos products amazingly enough!