However, the best point is the great support that is very responsive and really helpful, should you encounter any problem. Together with the very fair price this extension is a straight five star without any doubts.
The only real downside at the beginning is the documentation. It is something between real bad and simply non-existing. When you come to their document site the very first time you get the impression you hit the wrong link ;) - so hopefully the owner will take care a bit more about that in the future.
The CCK is built upon extra fields. You first define an extra field group and add the wanted fields and their types to it. After that, you assign this group to a category, and all content within this category will inherit the group. This concept helps organizing your additions to the standard Joomla content very efficient.
Unlike other CCKs like ZOO the extra field group is blocked within your content and added on the bottom, so your design capabilities are a bit limited.
The real benefit comes with the carefully integrated database support - you e.g. can easily fetch all articles from any single user, something I felt should be trivial however is found only within K2.
Joomla content, categories and users can be easily transferred to the K2 universe. Unfortunately, there is no integrated import/export tool for/to external data sources. The only import/export tool supporting K2 together with Joomla 3.x I found is Hexdata. It offers great possibilities like importing relations, however its current version has a bug and is not working with K2 at the moment.
K2 comes with its own tag system, that is a bit outdated now that Joola 3.x has arrived. Maybe this hopefully will be updated in the future.
K2 is rock solid and can be used on productive sites without worry. This makes it the first choice over SEBLOD, that appears to be in permanent early beta stage ;)
The component offers four different media categories - books, music, videos and games. This basic set is only expandable with sub categories, that are treated like their main categorie, so if you want eBooks or audiobooks, they would be a sub category of books and music. Also implemented is a solution for lending books, this makes it suitable for small libraries like in schools or churches. The real killer feature is the media information import from Amazon. However, it will not work in many cases (there is a longwinded explanation in the documentation why - the bottom line is that it will work for English, but mostly not for other languages). For me, it definitely did not work.
The component is built on top of VirueMart and acts quite separated from your Joomla universe (media, articles, reviews and so on are proprietary), also if you want to fit it in your template, you'll find yourself in some tweak work.
Installation can cause trouble because it has very specific needs to your php implementation, and you might find yourself negotiating with your hoster to compile an individual php server for you, that can be very time consuming and expensive.
The documentation comes only with the Pro versions, so when you really want to try the component with all its features, you're forced to buy it. The only living demo I found is in cyrillic letters and not understandable for non-russian speakers like me, so it's not really helpful. There are more demo links, but those sites were down when I tried to look at them.
Support is responsive within 24 hours, however in my case only offered individual (and hence paid) customization work.
So in general, if you look for features like genres or book series, you'll be disappointed, also the vendor is not planning to implement them in a future release (said it's too complicated), and you might better go for a content builder like K2 or Zoo. Other than that, when you can live with these limitations, it's a good choice.