Would e very intrested if this is possible out of the box. I've had to put in custom auditing in order to achieve this. 

What if you query the out of the box Activity Desintation Instances SmartObject and filter on the users network ID.  Note you will need the correct permissions to query across all process instances. 

Hi Tim, thanks for the reply.

This works!

One more question. I have to show only the process instances that user did an action on and sent them forward. Am I thinking correctly that I need to filter Activity Destination Instances List by Destination user, and if the Finish Date is not 1/1/1900? Or maybe if the Final Action is not empty?

Don't use the Finish Date since if someone else completed the work then the Finish Date will be that date and the Status will be expired. 

A simpler way would be to filter on the Status=Completed and Destination as that will also show items that they finished which sounds like what you want.

In my Activity Instance Destination List, all Status fields are empty, so I can't filter on that.

If the user did an action on the process instance, then the Finish Date and Final Action change. Is it not meant Finish date of the action that this user did? Like every action has it's own Finish Date, saying when the action was performed? The process goes to next user in the workflow, but still, the Finish Date for this process instance for this destination user changes. Did I understood wrong?

So I'm not sure why the status field is showing up blank as I couldn't reproduce that in my tests.  Are you using the SmartObject Tester tool to test?  What version of K2?

As for the 2nd part of your question I'm not quite sure I understood the wording:

If an activity has a single slot (only person needs to complete the task) then the Activity Destination Instance finish date for a destination user is the finish date for the activity.  If you require multiple people to complete a task (multiple slots) then they'll each have their own different Activity Destination Instance finish dates.

Could the Status field be blank since it's a development and incomplete process? I have placeholder event at the ending.

How can I use SmartObject Tester tool to test it?

K2 version is:

K2 blackpearl 4.6.8 (4.12060.1600.0)

K2 for SharePoint 1.0.1 (4.13350.1645.0)

K2 smartforms 1.0.7 (4.12165.1635.0)

K2 smartforms Control Pack 1.0.1 (4.13180.1670.0)

Users having their own Activity Destination Instance finish dates should be ok for me, since I don't care for activity slots for other users. I only want to see the process instancess this user did an action on (completed the activity, sent them forward in the workflow).

Run the smartobject in the SmartObject tester tool and see what it looks like there.  The tool is a must have for K2 development work.  It'll save you a lot of time.  Here is a link to a video that shows how the tool works.  http://help.k2.com/displaycontent.aspx?id=6243

When a process instance starts the status of the activities will automatically be given a status whether its in developement or not.