I also faced this issue and came up with a solution that captured the Activity Instance ID in a data field and then used the Workflow ReportsActivity Instance Destination SmO to return who completed the task (Destination), by passing in the ProcessID, ActivityInstanceID data field and 'Completed' for Status.  I then passed this Destination FQN value to the URMUser SmO to get the DisplayName.  This worked a treat, but would occasionally fail retreiving the user as the data had not been written to the relevant K2 log table.

Then I found this: http://community.k2.com/t5/K2-blackpearl/How-to-get-the-user-who-actioned-the-user-task/td-p/53443.  Yes it's Visual Studio, but only 1 line of code.  Just remember to create the datafield before attempting to assign a value. 

I can now get the user details into a datafield and use that value whenever I want.

We did the same thing without code by putting the smartobject call in the next activity and just putting a Preceding rule to start after 30 seconds. We also put a loop in to wait another 30 seconds if it was empty.

I have not found a way to do a preceding rule in designer but you work around this by assigning an activity to say the k2 account (don't send an email). Then escalate that activity after a minute and do the smart object call to get the data.

I use K2 studio but I train business users to use designer so I have had to do some very interesting work arounds including doing more heavy lifting with smart object calls in the form.

I hope this helps

This is great guys! Releived to know I'm not alone for one and sound suggestions, thank very much.