We just installed J-Comments along with the K2 Plugin (K2 JComments v.1.1). The conversion and everything went great :-)
One issue we ran into was the the comments counter on the category/user item lists continuing to report values from the K2 comments system instead of JComments.
This can be fixed with a quick change to /plugins/k2/jcomments.php. The behavior is caused by the K2 itemlist passing it's category params to the onK2CommentsCounter event instead of the params for each item. Not sure why K2 is doing that, but changing the plugin to reference the $params structure associated with the passed $item appears to always access the desired parameters.
To fix, find this code (in /plugins/k2/jcomments.php).
function onK2CommentsCounter( &$item, &$params, $limitstart, $profile='' )
{
$result = '';
if ($params->get('itemComments')) {
And change to.
function onK2CommentsCounter( &$item, &$params, $limitstart, $profile='' )
{
$result = '';
if ($item->params->get('itemComments') ) {
That seems to do the trick with all the views implemented on our site.
(Thanks for the great component!)