The answer to this question may vary from developers to developers. I truly believe that it also depends on the kind of programming language they use for developing applications. For example, I have heard many times from my fellow Java programmers that they couldn't live without the Eclipse IDE, that they will not be able to write a working Java program with the help of a simple text editor. Predominantly being a Python developer, I use Emacs for most of my coding. I do not feel code completion or any other fancy IDE features are required for coding in Python, atleast for what I'm doing. The only difficult thing I encountered using just Emacs for coding is debugging the code I've written. For most of the time
pdb to debug my code. This could be painful sometimes. Although there are some tricks in Emacs to do the debugging, it didn't seem neat enough to me.
That's when I found PyCharm IDE for Python. They had this program where they provide free licenses for the commercial version of PyCharm for doing opensource projects. I applied for one mentioning my voldemort project and I got one free license.
PyCharm comes with a brilliant visual Python debugger and I found it really easier to debug the code using breakpoints and variable watchers. I spent a month or two coding on PyCharm and it really helped me code faster. One of the main reasons I love Emacs is that I could do all the tasks just by using the keyboard. Be it cutting & pasting text, switching between buffers it was all easy. There was also this good feeling of using just the keyboard. After using PyCharm for quite some time, I felt like using the mouse a lot for most of these taks. Another advantage of using a text editor is that you will get to know the programming language a lot better since there are no autocompletion or any other IDE features. You have to write the code on your own. Then there are things like version control integration for IDEs which I believe eases the effort for most of the people. But I belive they conceal a lot of activities from the user. I prefer doing the version control activities by myself on a terminal rather than handing it to an IDE. In this pypix article some of the world's top Python developers talk about the tools preferred by them.
So what developer's should ultimately use? Hmm, I believe, there is no concrete answer. Nowadays, I use Emacs for relatively smaller projects and PyCharm for the behemoths. What do you think? What are you using for development?. Give your opinions/suggestions in the comment section.