The 650 size and weight is good for me - 90% of the time. From a usability viewpoint, I like the size - I'm 6-3"=big hands, so tiny phones are a pain for me.
As far as carrying it, for business use with a belt holster style case, or in a coat/jacket pocket, it's fine. It's OK in your pants pocket without the case, but it is slightly bigger and heavier than ideal. It usually doesn't bother me, but in your jeans pocket, if it rings, you WILL answer the call just getting it out of your pocket, by accidentally pressing the touch screen - and sometimes you don't want to take a call after you see the caller id!!!