I have the X. The only bad thing about it is it tends to RUN OUT because I use it All the time!! I've actually switched to the removable (vs the Permanent) since I use it mostly for cricut cut letters, and If I put the letter down wrong, it moves easily. and, it stays put with the page protectors on my pages.
I also have the 500, have had it so long and used it so much it's no longer working well! I would get the 510 if I felt I need it, but the X does what I need.