KDE Ships Plasma 5.3

I definitely agree.

I am sure that Plasma 5 will be very nice once it is completed, but so far it still has a non negligible number of problems and, most important, it misses a lot of functionality that existed in Plasma 4. Note: this is by no means as bad as the 3 -> 4 transition, still Plasma 5 will probably need another year of work or more to be on feature parity with 4.

So, if you are on 5.2 moving to 5.3 is a good idea. If you are on 4, stay with 4 for the time being... /if you can/.

The real issue is that Kubuntu has chosen to be Plasma 5 only for Vivid. They say, that they give you the option of staying with Plasma 4 with the LTS, but this is not a real option. Who updated from the LTS to Utopic last October is now forced to Plasma 5, that - at the current time - is still a big regression against Plasma 4. There is no Plasma 4 PPA for vivid (for some reason it is indicated as 'socially' unacceptable) and there is no infrastructure for downgrading to the closest LTS.

All this is very unfortunate for KDE because Kubuntu (which is one of the largest KDE distributors) is actively making easier to move from Plasma 5 to other desktops (from people I know and work with, cinnamon is the usual destination when coming from KDE) than from Plasma 5 to the full featured Plasma 4.