I've had three iPhones: 3G, 4, 4s. With my 3G, 18 months into owning it I upgraded it to the last iOS (4 if I recall correctly) and the phone became unusable.

I've had three iPhones: 3G, 4, 4s. With my 3G, 18 months into owning it I upgraded it to the last iOS (4 if I recall correctly) and the phone became unusable. Every time the phone rang, I would be swiping to answer the call and it would not respond in time before the call would go to voicemail. Every other function on the phone was also slowed down to the point of not being realistically usable.