I think writing a pre-established character from a canon source etc, such as comics, anime, tv shows etc, can be an interesting exercise. Again, how well someone can do that will vary from one person to the next wildly, so impressions of it can differ greatly, as Prince said. I think that a good majority of people would like to avoid the use of pre-established characters just to help avoid potential bickering and drama that might unfold in an ooc with regards to whether or not that character is being written accurately.

I also find it an interesting writing challenge to utilise and write other characters within my rp whom belonged to players who have dropped/left the rp, and to see how well I can come close to their established character and their rper's own writing style and interests, and keeping them incorporated into the rp and plot (as opposed to just killing the character off). In comparison to pre-established characters from comics, tv etc with so many different interpretations it's a lot easier to get accurately close to the character, but still a challenge that requires a lot of thought in itself. And one that I've always found rewarding to see when players who have dropped out pop back in to message me sometime later and thank me for the use of their character and how they were portrayed.