The only minor complaint I have is the wallpaper downloader still does not find any wallpapers, complaining of a network error, despite the network being very much alive. I ended up downloading one of my own photos and manually setting that as my wallpaper.
Also, the Plasma desktop did crash once or twice and I had to kill Xorg and restart KDE...
Muon is KDE's default software manager these days, but I'm not really that keen on it, and I often use the command line or Synaptic. It's purely personal taste. I installed kubuntu-restricted-extras and VLC as per usual for the extra codecs and VLC's advanced features. I also installed Filezilla and downloaded and installed Chrome.
All Ubuntu derivatives have a Restricted Driver Manager for proprietary drivers, on this Latitude D520 there's one for the Intel CPU microcode firmware.
Overall performance wise it is not too bad on this machine, definitely feels quicker than KDE 4 did on Kubuntu 14.04. However, this low end machine is not really quick enough for KDE and Chrome even with 3GB RAM! Probably best to use Firefox instead if you can or install a lighter weight derivative such as Xubuntu that uses the lighter weight XFCE desktop. KDE Plasma 5 though is shaping up to be a great desktop. I shall try other distros with it, to see if they have less bugs than Kubuntu does and hopefully it should be well polished by the time the next long term release of Mint comes along since that is what I run my main workstation on.