I've just swapped to one of these from an N95. I got fed up with the bulk of the N95 and wanted something more pocket friendly, which the 6650 is.
Comparing the two: obviously the camera is nowhere near as good but then the N95 camera was pretty rubbish (good pixel count, horrible delay between pressing the button and the picture being taken) as well.
HSDPA, which is what I use it for most, works perfectly.
GPS is a revelation compared with the N95. I quite often get a fix indoors which the N95 never managed.
The FM radio actually works.
Battery life is much better. Uses the same charger so I now have a spare.
Comes with a useful set of apps that run using the cover display so will work while the phone is closed. The music player was mentioned in the review, but there is also a stopwatch (which multiple split times) and a count down timer which is really useful.
Cons: headphones are really uncomfortable - they don't fit my ears and keep dropping out. Speakerphone mode is pathetic - the single biggest weakness for me.
Watch out for the usual T-Mobile left-hand right-hand non-aquaintance: they have an offer of Combi 30 contract, free 6650, free web'n'walk and free bluetooth headset at the moment. They were fine about letting me switch to this contract and about letting me pick the phone up from a shop (rather than have it posted which would have been inconvenient). But the shop said the bluetooth headset was for new customers only and I would have to contact T-Mobile. T-Mobile say that it is for all customers and I have to go back to the shop. <rhetorical> Just how hard is it to run a simple business? <rhetorical/>
Happy with the phone though.