ATG is an outstanding package. It is highly extensible, completely skinnable and very scalable. It is used by many of the largest online retailers, has a strong network of partners and support is quite good. ATG provide training courses for developers and architectural consultancy services. It includes integrations with advanced search, CRM, trouble ticketing and many allied functions.
All that having been said, it is closed-source, obtuse, complex, expensive and has a steep learning curve. It uses a proprietary ORM that mimics Hibernate and other language constructs that parallel J2EE or Spring such as dependency injection, but in it's own special way. Don't assume that your average J2EE or Spring developer will be able to pick it up quickly.
You do need to be careful of some of ATG's implementation partners, some of them are stacked with developers that have barely completed the standard ATG training program and you would be better off training your own staff. Check their references and make sure you actually speak to past clients.
Standard disclaimers apply, I have no affiliation with ATG, caveat emptor.