I don't know much about how Safari is for developers, but in my experience it is currently the best browser for the mac. The general performance of the browser (don't know about js benchmarks) and energy usage are a lot better. Also js based drag and drop is remarkably responsive, a lot more so than Chrome and Firefox. It does have a weird thing where it doesn't load Google sites for me sometimes.
Refering to Safari as "the new IE" isn't a comment about whether it's a good browser or not. It's because Safari is lagging some way behind the rest of the browsers when it comes to implementing new features based on standards. The problem is best illustrated by CanIUse.com ... http://caniuse.com/#compare=firefox+40,chrome+44,safari+8 ... Scroll to the bottom and look at the "Mixed support" table. Lots of green in the Chrome and Firefox columns, while there's lots of red in the Safari column.
Why this is a problem is that it means the web will stagnate. People will do new things, but only within the limits of the old technology. Many of the things Safari lacks are trivial to work around (an input color picker for example .. it's easy to do that in JS). Other things are much more important - picture elements and HTTP/2 being good examples.
