I have been using Vim for years (and was quite adamantly against Emacs). I work as a system administrator and for me, vi is one of the main tools of the trade, since it is on every system. On Linux systems you will mostly get Vim installed as the default vi, so learning and using Vim was natural. Most of my editing tasks were involving changing configuration files and writing relatively short scripts. Almost no debugging was involved and there as debugging, it was mostly just run/observe errors/fix script/run again cycle. For this type of use, Vim is perfect. It loads fast, so you can actually quit it every time you are done with editing and most testing/debugging can be accomplished by switching to a terminal window (or even better to a terminal window in a screen session). It is only when you start spending significant amounts of time writing code, Vim deficiencies start coming to light. What deficiencies? There are two main ones.
Vim is bad at communicating with external processes
While it is, of course, possible to run shell commands from Vim and even pipe data in the vim buffer, this is not enough. You need to be able to properly interact with a process such as a debugger. You need to send commands to it and capture their output, not run them and forget. Emacs is excelent at this, but Vim either has built-in support for a particular program (like gdb) or you are either out of luck or you will need a lot of hacking (like vimclojure).
Vim is not very good at editing multiple documents
Well, while this is not exactly true, Vim supports opening multiple files and recently added tab support, it is not as convenient or feels as natural as in Emacs. Multiple file support in Vim just feels awkward.
Extending Vim is a pain
Vim internal scripting language is strange, scripting with other languages compiled into vim, such as ruby or python is limited and not very portable. While many consider LISP to be strange, I find it to be not nearly as strange as vimscript.
Emacs is very good at communicating with external processes
So, you get a lot of benefits of the underlying OS right there in your editor. You also get much better integration with compilers, interpreters, REPL environments etc. You can use IRB and iPython or many other interactive dynamic language environments right out of the editor and get symbol completion and many other niceties. You can use programs like ssh, telnet or rsync to edit files on remote systems. There are too many uses to enumerate here, but I think you get the point.
Emacs is easy to configure
While originally you would have to configure Emacs by writing things in Emacs LISP, it is no longer required. Recent versions of Emacs sport very powerful customization interface, that allows you to change a lot of different aspects of the editor by pointing and clicking on things.
Emacs is old and the community is obsessive
While Vim has been around since 1991 and only got proper scripting support in 1998 (some would say in 2001), Emacs has been around since the 70's. And during these 30-something years, many talented people attempted to teach Emacs to do just about anything you could possibly imagine. So, if you want Emacs to do something, chances are, someone somewhere wrote a cute little bit of lisp that does exactly what you want.
LISP is good for you :)
And if Emacs is not doing something you want you can change just about anything. And you should. Cause anyone who calls himself a programmer should know at least a little bit of some lisp-like language and it might as well be Emacs LISP. It will alter you perception of reality, open your mind and chakras, walk your dog, neuter your cat and return your library books on time in under 10 lines of code.
But I am so used to Vim
Emacs has a mode called Viper, that makes Emacs behave in Vimish way. It has different levels, in order to gradually phase out your Vim habits. If you tend to enter cold pool by first dipping your little toe, you might want to start with Viper. I am more of a dive, head-first, while screaming obscenities person, so I do not use it.
But Emacs takes forever to load
Well, first, it is not true. A simple Emacs setup loads as fast as simple Vim setup and a complicated Vim setup loads as slowly as a complicated Emacs setup. And at that Emacs has autoload ability that allows you to only load minimally required stuff at the startup and load the rest when it is actually required. And Emacs LISP can be byte-compiled to speed up loading times. And in any case, Emacs is more of a programmer's editor, not sysadmins (I am having my doubts, but so I heard), so it is not really intended to be closed after every edit. It is intended to be loaded once at the start of the day and never stopped again and possibly stopped when the work is over, but not necessarily.
But all those parentheses are awful!!!
No, they are not. They are beautiful. And if you let Emacs do the indentation and turn on highlite-parenthesis-mode, they are even more awesome. And anyway, I think a person who is used to typing things like :g/^"foo.*?"/d and :s/^foo\(.*\)bar$/bar\1foo/ shouldn't complain about syntax.