I just upgraded my Sony Ericsson K750i to the latest firmware version: R1CA021. The actual upgrade took less than 20 minutes but I spent more than half an hour downloading the Sony Ericsson Update Service because I was also downloading a ton of podcast episodes that time.
To know the firmware version of your Sony Ericsson phone, do this (for < push the joystick to the left, for > push the joystick to the right): >*< <*<* After doing that, click on Service Info and then click on Software Info.
If you’re still using R1N035 or R1NA005 or R1AA008, then by all means download the latest firmware-it fixes the irritating USB transfer bug. If you already have R1BC002, I’d hold off for a week or two.