It can be a major point of stress for all concerned. The subjects of the letters wonder what we will write. The writers of the letters wonder (in some cases) what to write. Some applicants worry that some letter writers won't submit letters of recommendation on time (if ever); some applicants are justified in this worry. Some professors get annoyed when students ask them to write letters of reference on very short notice and/or provide disorganized, incomplete information.
And has any professor, during the year in which they were being considered for tenure and/or promotion, wandered around a conference feeling paranoid about who was writing their external letters and wondering what they were writing?
And so on. But write them (and read them) we must, in great profusion.
If I had to rank my preference in writing letters of reference, from don't-mind-writing-them (= best case) to don't-like-writing-them (= worst case), my list would look something like this:
top = best (in this relative scale); also: list assumes that I have agreed to write a letter so it does not include the worst case scenarios in which I would not/could not write a letter
- grads/postdocs applying for jobs such as faculty positions
- undergrads who are applying for something (grad school, internship, job) and who worked closely with me on a research project
- tenure and promotion letters for people who are great and whose work I know well
- colleagues being nominated for awards they deserve
- undergrads who are applying to something and who only took 1 class from me and about whom I know almost nothing except what their grade and maybe where they say in class but I feel obligated to write a letter in cases in which the student has no better options for letter-writers
- tenure and promotion letters for people whose work I don't know very well* (this is at the bottom only because of the vast amount of time it involves; if we factor out time, the previous one in the list is my least favorite).
But let's not focus on the negative. Why do I "like" (relative term) writing letters for students (whom I know) or postdocs applying for things? I like it because in most cases writing such letters is a very positive thing to do. If you have worked closely with someone for a year (some undergrads) or more (grads, postdocs, some undergrads), you probably have a lot of things to say, and, if you have agreed to write a letter, presumably you have some positive things to say.
Despite the time commitment, it can be a very positive experience for the letter writer to think back on someone's research/education experience, pick out the essential points and examples, and write a well-crafted letter geared towards the specific job/institution to which the candidate is applying. In this case, writing the letter ends up being an affirmative experience, as long as you don't think about the cynical committee members reading 100s of these awesomely positive letters and as long as you don't have to write many many of these letters at any one time, in which case the personal hand-crafted letter thing falls by the wayside.
* Some colleagues and I recently had an argument about this topic: Should you agree to write a tenure/promotion letter for someone whose work you don't know well? A post will follow at some point with elaborations.