NO DESIGN FLAW - USER ERROR - Well Designed, Its design is INTENTIONAL
I would give 6 stars if possible. For only $25.00 this is an extremely well engineered item. I also have an ORIGINAL Sonicare Advanced and it is nearly 13 years old now.  I learned a long time ago how damaging water can be.  My brother in law and my sister got Sonicare brushes for themselves and me for Christmas over a decade ago. Theirs went out quick, due to water and calcium buildup. All my Sonicare products I take extreme care of.  When using, I try and never let water drip down the brush head.  After use, I remove the brush head, and store it and the handle upside down.  This keeps water from going down into the device or the threads and causing problems.  If not stored upside down, not only can water get into the item, but, calcium and contaminants can leak down and get in the brush head threads.  All can be resolved by simple care and upside down storage. Of note, my original Series A over a decade old, looks like NEW from pampered care, excluding the discoloration of age. A little care goes a LONG way. A little lack of care/abuse and anything will deteriorate. With the top of the line Sonicare Diamond Clean being over $200.00 versus the HX3351 at only $25, you can't expect bullet proof.  For $300 to $400 they could make this out of aircraft aluminum like LED Flashlights and make it 100% waterproof but you wouldn't buy it, the cost would be prohibitive.  A little care goes a LONG way, 13 years for me and counting.
Rating: 5 stars