CCK.

If you need something really weird, do it in your module. Otherwise, do it with CCK.

CCK 6 is going to behave more like Form API, so much easier to create custom elements.

Image field and image cache are very worth checking out. Imagecache will auto generate different sized thumbnails.

CCK Mistake: still need to plan. Think ahead, plot out your content-types.

Consider using CCK and creating select list instead of using taxonomy. This can keep the category or whatever actually on the node. Using CCK is often more programmer friendly, though taxonomy is still often most friendly for content administrators.

Uses taxonomy instead of node_reference to do groupings. This way it is a two way reference.

OO Drupal:
Class == Content Type
Object == Node
Attribute == CCK Field
Function == hook_nodeapi