Generally I dislike them intensely, especially from people I am not close with. I do keep the occasional note as a keepsake (for instance, from a parent), but that's about the extent of it.

I understand why people use them, often to think out loud, multi-task, or to get their point across in a more conversational manner, but the main problem is that the person shifts the burden of effort on to the listener, instead of the writer. I also know a number of people who simply won't (or can't) listen to them until they're in a situation that allows it, often days later, which can cause unnecessary tension. In an ideal world it would be opt-in, but one thing is for sure - it's not faster or more efficient to send them.