I used Vim for 10 years leading up to delving into Emacs 2 years ago. I have a reasonably fresh recollection of just how my productivity curve modified over time.
My points are all conditional, YMMV depending on your strengths and experience.
If you have used Unix and the command line long enough that you are familiar with C-a, C-e, C-n, C-p, C-k, C-y, etc as they function on the shell, it will not take long to transition to using those same bindings (the defaults) in Emacs. I recently discovered that XCode uses these bindings as well.
If you are comfortable with an always running editor, tending buffers (like you would browser tabs) and thus living in the application (like you would with Web2.0 apps in the browser), Emacs will likely show immediate productivity enhancements.
If you generally work in projects of many related files, this persistence pays some added benefits in maintaining context to that buffer. Each buffer is contexted at its open file allowing for convenient use of various productivity boosting tools for your that project (like grep-find, eshell, run-python and slime). This coupled with text completion, yasnippets, etc start to look a tiny fraction like an IDE although ad-hoc and heavily individualized by your configuration. This is apart from more civilized Emacs IDE-like services like ECB.
My productivity took a hit initially as I typed "jjjkkk" constantly Esc-Esc-Esc-Esc for the first week or so. The following week I cautiously started using the right navigation keys. Then I discovered the configuration file... Honestly, if I had had Emacs Starter Kit from the start, I would have said my productivity slowly worked back to parity over the 3rd-4th week but I did go down the config file rabbit hole. A co-worker of mine, though, has just transitioned from vim to emacs and he just grabbed the Starter Kit and he is on his way. First week in and he seems comfortable and enjoying all the surprise benefits (that sensation will probably last a decade).
Finally, if you make mistakes you will immediately gain productivity (and confidence) from the circular kill/yank-ring and undo-ring. I am also personally a fan of region specific undos.
My short answer is Yes it is worth taking 3-4 weeks of a diminishing productivity-hit to learn Emacs. Even if you decide you prefer a streamlined unix utility combo over Emacs for development you will derive from it an education widely applicable beyond the editor.
