We have built a shop and are currently using this method for relating products:
Now although it does the job, it's a little cumbersome for the (backend) users to administer. All of a sudden they aren't just on another tab of the product, but are having to drill into blocks.
Parts of the system works really well, the blocks gracefully adjusting the style for example, but the way they have to add/amend related products seems like it's been tacked on compared to other product features.
The other big problem with this approach is permission levels. We utilise different user groups to do different things, one being a set of permissions to administer products. However if we wish that user group to be able to administer related products it appears we have to give them the Look and Feel>Edit Blocks privilege which means they can then edit/delete/adjust any of the blocks on the site. Obviously something that could be dangerous and not required for someone who administers products.
Thanks in advance.