The EAV model is great in theory and is probably great in practice _if_ the backing software has been _designed_ with EAV in mind.
In the context of Magento, it is a real _nightmare_ and is one of the major contributor of the slowness of the Magento platform (at least for magento < 2.0).
The reason for this slowness is that in a relational database, the EAV model makes it so that e-v-e-r-y s-i-n-g-l-e SQL query is one gigantic query made of tons of JOINs.
To give you an example, querying a product in Magento may need to join no less than 11 tables!
catalog_product_entity, catalog_product_entity_datetime, catalog_product_entity_decimal, catalog_product_entity_int, catalog_product_entity_gallery, catalog_product_entity_group_price, catalog_product_entity_media_gallery, catalog_product_entity_text, etc, etc.
In order to fix this issue, the Magento team created what they call "flat tables" which are tables that are created by querying the database with an EAV query (i.e. the query with a million joins) and putting the results in a table with as many columns as there is attributes being returned by the original query.
In theory choosing to use EAV was an amazing idea. In practice, this idea did not scale for large Magento stores and it has made Magento hugely complex, slow and hard to use.
We use Magento at betabrand.com and I can confidently say 90% of the slowness of our website is due to Magento's EAV tables and we have spent a humongous number of engineering hours optimizing this.
Are you on Magento 1 or 2? We've been building a site in Magento 2 and while the number of joins, hassle with re-indexing performance, and lack of null vs zero vs empty string handling in the flat tables are all very annoying, the actual site is quite fast with all the caching turned on on a server with enough gerbils behind it.
My main complaint with Magento 2 is with the feature that is it's biggest selling point: its flexibility. The fact that any public method on any class is wrappable/replaceable, and that any class in the system can be replaced wholesale, and any javascript or any template in the system can be wrapped or replaced, from anywhere, by any module, at a distance, just makes the whole thing a huge cluster to deal with once you get any number of 3rd party modules.
