K2 component is easy to use and easy to set up. It is mighty, you can built many powerful things with it. Thats the good side.
The dissatisfied side is: if you need help from K2 community or developers, you lost. K2 team doesn't care about community. Very very important bugs with thousends affected users, like duplicated urls issue, are not answered from team till year. The answers, which was given, was just "go away"-answers, they gave no real solution. Active users are just spamers for K2 team, which must kept away from posting through filling captchas, even after registering and more then 40 topics. In the forum with more then 13k threads is 1 (one) active moderator, who isn't knowledgable enough, like team members must be.
Yes, K2 is free. You can use it, there are many good extentions for it too. If you good know, how k2 works, you will be happy with it. And you can let it be, if you plan a project, which must earn money and you need developers help.
I use k2 at a commercial project and i'm really scared, what will happens with the project, if my knowledge will be once not enough and i haven't somebody to help.