Used latest K2 for Joomla 2.5.
1. The transfer of items from Joomla was not good - unwanted junk was transferred but the new categories that I created did not transfer. But hey this is small! Categories I can retype manually.
2. More important - comments do not work as promised.
Any unwanted user-spammer can type in his URL link and there is no way to switch it off, unless you ban the user. I did not find an option to forbid outgoing links.
While searching for the solution in K2 comments options - modules etc ( did not find any option to switch off URL from unregistered user) but found CAPTCHA option. Turned CAPTCHA on and it does not work at all - on the page there is claim to put in 2 words , but no words displayed. So no user can use it at all now.
Even worst - that CAPTCHA option disappeared form the module - WOW - not able to turn on, or turn off.
Too many bugs.
Tried to find info on the K2 forum and see that similar questions about comments and CAPTCHA are sitting there for months without any forum support.
So, K2 is for heavy programmers who love to struggle with bugs and debug things.
1. Migration from the default Joomla! article system is done manually. It's not automatic (why would it be). If you saw "junk" coming into K2 then junk was placed in your Joomla! articles. Of course new categories that you create in the Joomla! article system will not be reflected into K2 unless you import them first.
2. Comments work EXACTLY as they should. You just need to have a clue of the technologies involved before using them. E.g. for reCaptcha to work, you need to get to keys (free) from recaptcha.google.com. Had you read the tooltip explanations on each K2 parameter you would've known that of course. Got a problem with real human spammers? Cut them off by enabling the built-in StopForumSpam.com support and/or enabling comment moderation. K2 not only has the best possible anti-spam protection, it also protects your entire Joomla! site from spammers when you enable the built-in StopForumSpam.com options.
To recap things, you've mentioned no bug so far, just misconfiguration from your side. And if you got no response, that's because the answer was sitting right there for you in the K2 parameter tooltips.
Lastly, if K2 was for programmers only, a) it would not have 750.000+ downloads so far and b) it would not be used on high profile Joomla! sites worldwide...