Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Warning - Nasty Gunk Buildup

I had owned an Oral-B Sonic Complete S-320 that finally was no longer able to charge so I ended up replacing it with the Sonicare Essence. The sonic technology in both toothbrushes is similar so there was no real change in the actual functionality of the toothbrush. I much prefer the sonic vibrating toothbrushes to the Oral-B vitality types that just spin.The one major difference with Sonicare is that the toothbrush heads are much larger and more expensive. Even worse is the fact that the way they screw onto the base unit makes it very easy for water to seep in and for really nasty smelling gunk to build up inside the handle. I've gotten so frustrated with this gunk and with the toothbrush heads unscrewing themselves that I'm going back to order the discontinued Sonic Complete and hoping that it will help me to get that funky smell out of my head.