The majority of my rejections have been form, especially on the last book, but I did receive one that apologised in great length for taking a while to reply, as they'd been moving office or something but at the bottom said they hoped I'd found a placement for the book in the time I'd been waiting for their reply. Another form one, I suppose, but for some reason I felt a little better about that one than the 'sorry, we have lots of books to look at' ones.
My absolute favourites were the ones where something had been scribbled onto the form rejection though. I got one ages ago from a literary agent, form rejection, with some chatty lines about how she used to live in the same part of London as I did (at that time) and what a nice area it was. She said she wished me all the best with my work but she had to turn it down as she 'just didn't like Elves'. That made me laugh.
Another one I got from I think Orbit Publishing had some notes scribbled on it saying not to pay anyone for publishing, but to keep trying. Just when there's something written in pen or pencil on the letter, I feel it's a bit more personal.