I was trying to create a dashboard for every user that lets sumarize the open issues for this user, to avoid to many levels for a user to find how many issues has.
Trying to get the filter and the argument "assigned" I have only found that there's a patch (thank's to Sean B Fuller [http://drupal.org/node/396670]) that acts as a boolean (assigned or not), and that lists the issues that are asssigned to the logged user.
I have hacked a little to get a filter that lets you choose the user which this issue is assigned, and also the argument to refer to list user's assigned issues via path.

I have copied and modified the argument and filter handler from user views submodule, I hope that you found it useful.
To activate this:

these are the differences in project_issue.views.inc :

me@nowhere.noplace [~/www/drupal/sites/all/modules/project_issue/views]# diff -u -r  project_issue.views.inc /tmp/project_issue/views/project_issue.views.inc 
--- project_issue.views.inc     2010-07-22 06:32:42.000000000 -0600
+++ /tmp/project_issue/views/project_issue.views.inc    2009-06-17 21:48:53.000000000 -0600
@@ -158,25 +158,6 @@
       'label' => t('Assigned user'),
     ),
   );
-//PATCH per llistar els issues assignats per lusuari.
-  $data['project_issues']['assigned_user'] = array(
-    'real field' => 'assigned',
-    'title' => t('Assigned to this user'),
-    'help' => t('The issue is assigned to this user.'),
-    'argument' => array(
-       'handler' => 'project_issue_handler_argument_assigned_user',
-        'name field' => 'assigned user',// display this field in the summary
-    ),
-    'filter' => array( 
-      'handler' => 'project_issue_handler_filter_user_assigned',
-      'title' => t('Assigned User'),
-    ),
-    'sort' => array(
-      'handler' => 'views_handler_sort',
-    ),
-  );  
-//Fi PATCH
-
 
   $data['project_issues']['sid'] = array(
     'title' => t('Status'),
@@ -239,15 +220,6 @@
       'project_issue_handler_filter_issue_version' => array(
         'parent' => 'views_handler_filter_in_operator',
       ),
-      //PATCH PER LLISTAR SEGONS ASSIGNATS A USUARI
-      'project_issue_handler_filter_user_assigned' => array(
-        'parent' => 'views_handler_filter_in_operator',
-      ),
-      'project_issue_handler_argument_assigned_user' => array(
-        'parent' => 'views_handler_argument_numeric',
-      ),
-      //FI PATCH
-
     ),
   );
 }

And you have to include these two files in handlers directory: 

me@nowhere.noplace [~/www/drupal/sites/all/modules/project_issue/views]# cat handlers/project_issue_handler_filter_user_assigned.inc 
<?php
// $Id: views_handler_filter_user_name.inc,v 1.2 2008/09/22 23:41:14 merlinofchaos Exp $

/**
 * Filter handler for usernames
 */
class project_issue_handler_filter_user_assigned extends views_handler_filter_in_operator {
  var $no_single = TRUE;

  function value_form(&$form, &$form_state) {
    $values = array();
    if ($this->value) {
      $result = db_query("SELECT * FROM {users} u WHERE uid IN ("  . implode(', ', $this->value) . ")");
      while ($account = db_fetch_object($result)) {
        if ($account->uid) {
          $values[] = $account->name;
        }
        else {
          $values[] = 'Anonymous'; // Intentionally NOT translated.
        }
      }
    }

    sort($values);
    $default_value = implode(', ', $values);
    $form['value'] = array(
      '#type' => 'textfield',
      '#title' => t('Usernames'),
      '#description' => t('Enter a comma separated list of user names.'),
      '#default_value' => $default_value,
      '#autocomplete_path' => 'admin/views/ajax/autocomplete/user',
    );

    if (!empty($form_state['exposed']) && !isset($form_state['input'][$this->options['expose']['identifier']])) {
      $form_state['input'][$this->options['expose']['identifier']] = $default_value;
    }
  }

  function value_validate(&$form, &$form_state) {
    $values = drupal_explode_tags($form_state['values']['options']['value']);
    $uids = $this->validate_user_strings($form['value'], $values);

    if ($uids) {
      $form_state['values']['options']['value'] = $uids;
    }
  }

  function accept_exposed_input($input) {
    $rc = parent::accept_exposed_input($input);

    if ($rc) {
      // If we have previously validated input, override.
      if (isset($this->validated_exposed_input)) {
        $this->value = $this->validated_exposed_input;
      }
    }

    return $rc;
  }

  function exposed_validate(&$form, &$form_state) {
    if (empty($this->options['exposed'])) {
      return;
    }

    if (empty($this->options['expose']['identifier'])) {
      return;
    }

    $identifier = $this->options['expose']['identifier'];
    $values = drupal_explode_tags($form_state['values'][$identifier]);

    $uids = $this->validate_user_strings($form[$identifier], $values);

    if ($uids) {
      $this->validated_exposed_input = $uids;
    }
  }

  /**
   * Validate the user string. Since this can come from either the form
   * or the exposed filter, this is abstracted out a bit so it can
   * handle the multiple input sources.
   */
  function validate_user_strings(&$form, $values) {
    $uids = array();
    $placeholders = array();
    $args = array();
    $results = array();
    foreach ($values as $value) {
      if (strtolower($value) == 'anonymous') {
        $uids[] = 0;
      }
      else {
        $missing[strtolower($value)] = TRUE;
        $args[] = $value;
        $placeholders[] = "'%s'";
      }
    }

    if (!$args) {
      return $uids;
    }

    $result = db_query("SELECT * FROM {users} WHERE name IN (" . implode(', ', $placeholders) . ")", $args);
    while ($account = db_fetch_object($result)) {
      unset($missing[strtolower($account->name)]);
      $uids[] = $account->uid;
    }

    if ($missing) {
      form_error($form, format_plural(count($missing), 'Unable to find user: @users', 'Unable to find users: @users', array('@users' => implode(', ', array_keys($missing)))));
    }

    return $uids;
  }

  function value_submit() {
    // prevent array filter from removing our anonymous user.
  }

  // Override to do nothing.
  function get_value_options() { }

  function admin_summary() {
    // set up $this->value_options for the parent summary
    $this->value_options = array();

    if ($this->value) {
      $result = db_query("SELECT * FROM {users} u WHERE uid IN ("  . implode(', ', $this->value) . ")");

      while ($account = db_fetch_object($result)) {
        if ($account->uid) {
          $this->value_options[$account->uid] = $account->name;
        }
        else {
          $this->value_options[$account->uid] = 'Anonymous'; // Intentionally NOT translated.
        }
      }
    }

    return parent::admin_summary();
  }
}

me@nowhere.noplace [~/www/drupal/sites/all/modules/project_issue/views]# cat handlers/project_issue_handler_argument_assigned_user.inc 
<?php
// $Id: views_handler_argument_user_uid.inc,v 1.1 2008/09/03 19:21:30 merlinofchaos Exp $
/**
 * @file
 * Provide user uid argument handler.
 */

/**
 * Argument handler to accept a user id.
 */
class project_issue_handler_argument_assigned_user extends views_handler_argument_numeric {
  /**
   * Override the behavior of title(). Get the name of the user.
   */
  function title_query() {
    if (!$this->argument) {
      return array(variable_get('anonymous', t('Anonymous')));
    }

    $titles = array();
    $placeholders = implode(', ', array_fill(0, sizeof($this->value), '%d'));

    $result = db_query("SELECT u.name FROM {users} u WHERE u.uid IN ($placeholders)", $this->value);
    while ($term = db_fetch_object($result)) {
      $titles[] = check_plain($term->name);
    }
    return $titles;
  }
}

Once you do this it's important to sure that ALWAYS the first argument in issues per user view is "Node: User posted or commented" as it comes by default(it's bypassed including "all") in first path argument when you visit the page, for example ttp://communia.info/communia/project/issues/user/all/5, where all argument means all users, and second mean assigned to user 5.
If you remove this argument you will get an error refering to
$arg_uid = $view->argument[$argument_name]->get_value(); in project_issue.module, line 1690.
If you get this error comment this line and put:
$arg_uid = 18;
repair what I've said about arguments and uncomment line 1690 again and remove $arg_uid = 18;

I have no time to fix this annoying bug in the patch...

Hope you enjoy