I have been on Ubuntu Linux for six months now. I first use Version 10.10 and am not on Version 11.04 on a four year old HP Compaq 6510b which used to run Window Vista with the RAM upgraded from 1GB to 2.5GB.
Hardware. When I installed Ubuntu 10.10 the benefits were clear immediately. I found the operating system so efficient I downgraded back down to 1GB of RAM. I have another HP 6510b running Vista with 2.5GB of RAM. I place the 2GB of RAM in another HP 6510b running Vista. Ubuntu Linux with 1GB of RAM runs faster than Vista with 4GB.
Two finger scrolling works on the touchpad. I was able to output video at 720p via the VGA port to my LCD TV. Things I cannot do with Vista. Installing Ubuntu made what felt like an old slow machine to a new fast machine.
While most everything including WiFi and Bluetooth worked without any special configuration, I still have had no luck with the Fingerprint scanner and configuring a printer is still a pain.
The upgrade to 11.04 brought the new Unity interface which I love at a moderate increase in system resource use, I lost the ability output video at 720p now being limited to 1024 x 768, and the hardware mute button indicator light on the HP 6510b is a little buggy, displaying the the laptop is set to mute mode when it is not. The button itself works fine.
Software. Ubuntu has its own "App Store", but outside that installing software gets difficult. Even when you have version designed specifically for Ubuntu Linux installing is more than a download and double click affair. Installing Google Earth and Dropbox required searching on Google to get them working and entering commands via the Terminal.
Once you get them working, functionality might be below 100%. Sharing places via email on Google Earth does not work.
Summary. Ubuntu Linux is 90% of the way to being ready for the prime time. There really is no need to improve the interface or its functionality right now. It is at parity with everything else in the market, but hardware compatibility and installing of third party software is what needs work.