Pros
Gartner has a very strong name and reputation in the tech community, mostly from its research, on-the-spot analysis in the media, and the over-the-top events it hosts for customers.
Cons
Research is the king of everything that is done at Gartner. Essentially everything else is designed to support the growth and sales of research product. Consulting is a "third tier" activity that is supposed to amplify research for its customers, but since it costs more to produce consulting than research, it is often left to make its own way. There is no clear progression for staff, as it is a firm designed much more for a seasoned professional than junior staff. They do not offer training opportunities to staff, there is no clear mentoring, and no real supervision, except on a project by project basis.