I just restored my crusty, yellowed headlights on my '05 X5 with the 3M Ultra headlight restoration kit and damn they look good as new. The final step is to apply two coats of the included 3M clear coat. I think that will last a year or so before needing a reapplication.

Took me about 90 minutes for both headlights.

Based on a tip here, I am also considering buying pre-cut E53 headlight films from XPEL or Lamin-X for $60 to put on after restoration to preserve them even longer.