Electric toothbrushes. They really are superior to regular old brushes, but they tend to break down after less than a year and aren't exactly cheap. Ironically, the last time mine broke I replaced it with the cheapest one and it's lasted longer than the ones before it. Go figure!
I've had two electric toothbrushes in my life. The first one lasted for maybe 10 years. The breaking point was plastic degradation which occluded the internal electronics and destroyed de button to turn it on.
I think could have been repaired with the right materials. The repairability of the brand I buy is pretty good.
For anyone curious the brand is:
Tap for spoiler
Oral b
The electric toothbrushes are nice but the head replacements are too expensive and I've not find a suitable offbrand replacement. So I end up boiling the toothbrush heads several times to extend their lifetime.
Philips Sonicare. My first two developed the same problem: some connection inside came loose and the head would be loose and rattly. I tried opening them up and fixing them, but they were too bent out of shape. My third one ( a slightly different model) is going strong for some time now, so maybe I just got unlucky.