Valve seems to make mostly FPS type games. Not really a huge fan though I logged in a few hours playing Half Life 2.
Their games are very nicely polished which is something any consumer should appreciate. The reason I love them and keep going back though is Steam. They were the first to my knowledge to come out with a unified package for purchasing, downloading and organizing games while unifying with a nearly unobtrusive DRM all under one desktop icon. There is a reason why Steam has nearly killed brick and mortar stores. They just offer more, better, faster service than any real store can give. They did it first, and after a rough launch, did it best. They are also one of the largest platforms for indie distribution I have ever seen.
They absolutely deserve to have the strong position they currently have. As long as their business practices remain as they are I approve.
I live in fear of what may happen if a prick like the head of Activision or EA takes over though. they have a virtual monopoly and gaming may well receive a death blow if other publisher heads ever get control of the company.