Did a full wipe and setup on my Ultra 3. New Siri works fine with everything else, but it still refuses to set a simple timer when I say "set a timer for 10 minutes". New Siri has no issues going through my emails when I ask for something, through text messages, or turning lights on/off through HomeKit, but not a timer for some reason. Will have to wait until B4 it seems.
It's not as quick as the old Siri at the moment when doing simple requests; it feels about the same as the old watches when Siri sent even the simplest requests from the watch to be processed. B3 on the iPhone, the new Siri is definitely more snappy than it was on B2.