i like the steam UI, and it's where all my games are, like the previous users say...
it's so fucking easy to launch games through steam and delete\download as i wish since i don't have massive storage...
then there is the in game overlay, i use the browser to search for stuff related to the game when i just forgot about something (like weapon stats, or craft blueprints)
steam is almost perfect, just missing some features and stuff that could be improved but since i don't like to have all kinds of software installed and running, steam does the job
you can also put shortcuts, use the small mode, i don't use the big picture thing
but the store, feeds my shopaholicism.. every fucking day i have the reflex of going into the store looking for something i can't buy