I have. Most of my RP characters are based off of myself in some way or another, particularly a character named "Ryu" who hasn't really made much of an appearance on this site, but -all- of his experiences in his story are heavily based on things that really happened, albeit mostly in an MMORPG.
My character, Kiel, is also based ... very loosely off of myself, if I'm honest. Arguably a much darker and heavily fantasized / supernaturally influenced version.
As for your question: I know you addressed it, but everyone is different and some of the personal experiences people write about are very painful. It's easy to talk about a silly or stupid or random occurrence in your life, but sometimes the stories are about loneliness, abuse, anger, break ups, abandonment issues, trust issues, exc. How hard it is to write depends entirely on the individual writing.
And, as for how rewarding it is, that depends on the individual, too. If it's something you need to get off your chest and you express it well, a real-life based story can be very satisfying to complete. On the other hand, it can also be -too much- reality. Sometimes fantasy is just ... more fun.