I had it on my Z4MC from about 14,000 miles to 100,000 miles. Would have kept it going, but the same premium offered much reduced cover after 100k.
In my case, and doing 20k miles a year, it was worth it. I probably paid out close to £5k in 5 years (mine was the £0 excess, fully comp option at £86/month), but got about £15k of warranty repairs done (engine, diff were the main items).
If you're not doing a lot of mileage, then it seems to make sense to just put the money away for a rainy day.