My current phone (an antique by my normal standards - Nexus S) isn't rooted, because I had no need to. It's got vanilla gingerbread on it and it runs like clockwork and it's plenty fast enough.
My previous phone, a Vibrant (as an example) was a really good phone out of the box, but by rooting it, it was soooooooo much better.
I could run apps like WiFi Hotspot, that TMO otherwise blocked.
I could customize the user interface to suit me, outside of the box of the canned UI provided by Samsung/TMO.
I got updates and improvements on a weekly basis (within the framework of the released code rev), whereas Samsung/TMO hadn't released an official OTA as late as several months after release, despite several rather serious problems.
Improvements in the file system made a dramatic improvement in the speed and usability of the device.
Rooting is a case-by-case basis thing, depending on the device, the amount of community support available, the individual, and the quality and usability of the device without rooting.
There are obviously downsides.
With some devices, you can permanently brick your phone, even if you do everything right. I've never done this, but I've had a few scary moments.
If updates are too frequent, it can be annoying.
If the developer(s) aren't experienced, or on fresh builds, they can be very unstable, resulting in untimely lockups, and frequent reboots.
Overclocking is generally bad for the long term health of your device. They run much hotter, and have a much higher rate of failure.
I don't have the exact specifics, but a fairly significant portion of the early warranty return devices come back having been rooted and overclocked, and probably died running a bench mark program.
Now, back on topic...
I just discovered the Kindle Cloud Drive app for Chrome last night.
Pretty coolI only own a couple of kindle books, so I hadn't really looked for this, but I stumbled on a kindle book on Amazon last night that I wanted, and couldn't find anywhere else.