Can we install Ubuntu on any smartphone / tablet and how user-friendly is it?
Ubuntu has currently tested its OS for Galaxy Nexus phone.If you try to use it in your own phone,you might face problems as the kernel versions varies between different kinds of phones.
This is the reason why one can't get updates from Google even if they get stock Android in their phones.
There will be Linux phones coming out very soon, and there are Linux versions that can be downloaded and used on some Android phones, but because not all phones are made with only a few different forms of hardware (processor, GPU, cell phone radio chips, etc.), it is much more difficult to have the proper universal driver support for an easy installation process such as can be performed on regular computers. There are Linux versions that are made compatible (to a point) for many popular brands of Android phones, though these may not be the latest version.
As far as functionality, some issues are to be expected since driver support is not fully implemented for smartphones as they are for laptop and desktop computers. There are some Linux versions that are made compatible with some smartphones and most functions are implemented including phone calling capabilities, but I wouldn't expect a polished product. A polished version of Linux for mobile should be out when the Linux phones come out publicly, so developers such as the ones at http://www.xda-developers.com should be able to make it more compatible with more phones with better functionality. If you were hoping for a desktop-like experience, it may not be as smooth and software support probably would be very limited, at least until the official mobile version is out.
More information can be found here: http://www.xda-developers.com/tag/linux/ and here: http://forum.xda-developers.com/showthread.php?t=1585009
@Dalsan M, your response would be pretty confusing to someone who actually needed info on this. Ubuntu and Android are both Linux-based OS's, so it doesn't make any sense to say "there will be Linux phones coming out very soon," when there are already scores of Linux phones on the market. When answering a question, it's much clearer to use the name of the actual operating system we're talking about, whether Ubuntu, Android, Sailfish, Tizen, or whatever.
In the technical sense, you are right, but Android runs on the Linux kernal. I should have said Ubuntu phones are coming out instead of being so generic. The general public views Android as only Android, not Linux. Linux users, however knows that Linux is used to create the Android form as well as many others. I do agree that I should have been more specific for this question.
In principle, yes. The minimum requirements are 1Ghz Cortex A9, 512 RAM adn 4GB flash memory. You can find out more at http://www.ubuntu.com/phone/operators-and-oems. I can't comment on usability or user friendliness as I've not tried it myself. Also, watch out for driver problems! Many smartphones have specialised hardware (cameras etc) and there doesn't appear to be an HCL on the Ubuntu web site. Still, if your phone is not too precious to you, give it a shot