Judging by the large number of replies you have got, I am afraid nobody admits to having any experience. I have never heard of r_star trees, and couldn't find anything in Wikipedia.
Try getting your hands on a copy of Donald Knuth the Art of Computer Programming; on Amazon I found
* The Art of Computer Programming, Volume 4, Fascicle 4 - Generating All Trees - History of Combinatorial Generation by Donald E. Knuth
* Paperback: 128 pages
* Publisher: Addison Wesley; 1 edition (16 Feb 2006)
* Language English
* ISBN-10: 0321335708
* ISBN-13: 978-0321335708
I am afraid I don't know whether you want that part, or volume 3.
I went onto Google and found this project report, but it is written in C and is very hard to understand.
You are going to have to ask for more details, I think.