Hi Ross - I have had my hips for over 15 years, I am very active and had ONE dislocation, one week after a revision surgery. It was due to too small a head being used by the surgeon. Once fully healed (9 months) I have felt NO instability. According to my very experienced surgeon, dislocation after uncomplicated THR is extremely rare. I was careful to find a surgeon with an excellent record regarding post-surgical infection, since an infected joint is a bigger problem.
I believe the key to success is to do as much therapy as possible BEFORE and after surgery so the muscles are in peak condition and follow the guidelines during healing. The people I know who have had complications have either had previous injuries that were being corrected by or were found during the surgery. Another cause for problems afterward is failure to do the prescribed exercises because they hurt, are boring, take too much time...
Good luck.
Sue