While Chrome likes to emphasize how speedy it is, it is also sometimes a pretty bare-metal browser. But all the speed comes at an expensive — Chrome is doing less out of the box, which some people will say means, "Chrome is focusing on the essentials".
So because I'm thankful for said speedyness and overall painlessness, I also realize how I little extra bells and whistles I really need for a great browsing experience.
And in the end, being speedy and painless clearly wins.
I've been using Chrome for over a year now (or longer?) and aside from a couple custom extensions I wrote for my own pleasure, so far there only three other extensions I installed.
JsonView formats JSON into a pretty structure which I can expand and contract. It also adds some color. This extension is incredibly helpful to those dealing with web services which exchange data using JSON. A must-have if you recognize yourself in that group.
XML Tree is just like JsonView, but for XML. Yes, it's dead simple, but I also highly recommend it.
SpeedTracer could be called the equivalent to Firebug. But since Chrome already ships with a pretty comprehensive developer console (which gets you about 80-90% of what Firebug does), SpeedTracer really is the icing on the cake. Not a must have, but a nice to have.