Lubuntu is a fast, light, less resource hungry and more energy-efficient operating system based on Ubuntu but using the LXDE desktop environment that works great less-powerful machines such as netbooks, mobile devices and older PCs.
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By utilizing minimal software Lubuntu is able to use less resources and power.
Pro Part of the Ubuntu family
Being based on Ubuntu (which itself is based on Debian), there's a good amount of information and support to be found when searching for it, both on Lubuntu specifically and Ubuntu that also applies to Lubuntu. It also means that there are certain security and usability standards enforced and you can use Ubuntu .deb packages.
Pro Offers a long term support release
Lubuntu offers a long term support release with their 14.04 Trusty Tahr based version.
Pro LXDE desktop
Lightweight X11 Desktop Environment performs fast and conserves resources.
Pro Uses Openbox as the default window manager
Lubuntu (LXDE) uses the lightweight Openbox window manager to manage its windows. By using Openbox by default LXDE is able to remain lightweight as there is no built in composite manager in Openbox to take up resources.
Con Long term support not as long as other Ubuntu spins
Lubuntu has not always had LTS support. Luckily they do offer a 3 year LTS now but it is shorter than other Ubuntu spins that receive a full 5 years of support.
There is absolutely no reason to install it, since it uses the same package repositories as Ubuntu and you can transform any Ubuntu install into a Lubuntu install.
Con Lacks some window management features
There's no built-in way to snap windows to the sides, re-size them by double clicking the bar, or dragging to the edges.
Con No composite manager
Being a modern desktop many users expect compositing, even on lightweight systems. Without compositing there will be screen tearing, a lot of it. Though there is built in support for users to add their own compositors such as Compton.
Con Clock in panel is broken currently in LXDE
The clock panel applet does not work as expected in the newest versions of LXDE. If the clock is set to not show seconds in the display when waking from suspend the clock will not update until the time actually changes in the applet which means it can take up to a minute for the clock to update when seconds are not shown. The work around to this is to display seconds on the clock which allows it to update after suspend within one second. Sadly the clock does not display seconds correctly as it skips them by one to three at least once a minute, often more.
This is also not the first time there have been problems with the panel clock which clearly illustrates this to be an ongoing problem for the developers.