How common is this? Has anyone here been asked to do this? What did you write?
It makes me somewhat uncomfortable, as something about it seems cheap and dishonest...I'm also not someone who has an easy time talking about myself in a less-than-humble way ("get over it," I know).
I feel like, if I do this, I will drive myself nuts trying to concoct my ideal letter that I would normally dream of getting. What to do...what to do...
i've done this. its usually someone that wants to help but knows they will not be able to write a good letter based on time constraints or not enough information. is it lazy, yes, but it is also a golden opportunity.
don't aim for an ideal letter because it will sound bs-ed. the key is to make a few compliments, and back them up with specific evidence. this is what adcoms like to see. they don't like empty statements that have little value and don't set people apart.
think about your relationship with the letter writer, what they know about you, what they could easily know about you, and think about what is the best thing they could say. ex: "so and so has shown remarkable leadership skills in our lab by designing his own X project" "he has definitely been one of my top five most knowledgeable and effective students"
these are examples, depends on your relationship. this is very common because a lot of profs, employers, etc. get asked to do this a lot, and in all honesty this is a lot better than having them put your name in a 'mail merge' form document (which i have also had) that tells nothing of your individual accomplishments and qualities.
if you are not comfortable, perhaps you could express this to the writer, but you should probably just do it and not use it, but instead find someone else who can put aside the time to write a meaningful letter.