I'm a long time KDE user. I loved KDE4 and I've installed it on countless machines. After some cautious wait, I've decided to move on Plasma 5. Here is a brief description of whatI've found so far.

I have plasma 5 installed on three machines by now. Two old laptops (~6yr old) and an old desktop (~7yr old with some upgrades).

• Laptop 1: ArchLinux
  Plasma 5.3
  Intel l Core2Duo P8700 with chipset integrated graphics
  4GB RAM
  Samsung 840 EVO 256GB SSD
  1280x800 13.3"
• Laptop 2: Kubuntu 15.04
  Plasma 5.3 (upgraded from 5.2)
  Intel Core2Duo SU9400 with chipset integrated graphics
  3GB RAM
  Mechanical HDD
  1366x768 14"
• Desktop PC: Kubuntu 15.04
  Plasma 5.3 (upgraded from 5.2)
  Intel Core2Duo E6400
  ATI HD4850 Iceq4, open-source drivers
  6GB RAM
  Samsung PM800 (old) 64GB SSD
  1920x1080 22"

In general my experience has been very positive. On all machines Plasma 5 loads very fast, is responsive and lightweight and looks good. Some of the major features of KDE4 are missing, but I hear they'll reintroduce them (more on that below). I've been almost completely crash-free, and those crashes are application only so not a big deal. It uses more RAM than KDE4, but I do not see that as a real problem.
Overall I'm pretty satisfied, and I trust more features will be added. I was greatly disappointed not to find some basic things such as different widgets and desktop type for each virtual desktop.

Here below is a list of the bugs and problems I've encountered. I'll report them asap, but if someone can tell me some have already been reported, I'll be extremely grateful. All the bugs below are confirmed on all 3 machines when not differently specified.

• In KDE4, AFAICT, the dashboard was NOT a copy of the desktop widgets. Now it seems it is: I can't find a way to put widgets on the dashboard only and the dashboard is just a copy of the widgets on the desktop
• "Energy information", which is in KInfoCenter, should be reachable from the battery icon in the system tray
• A description should be added for the desktop types Hunyango and Haenau
• Bring the "different widgets on each desktop" option back! This is vital!
• SDDM login icon and User avatar are not in sync - if I change the user Avatar in Account details, this has no effect on SDDM (but has on the lock screen icon)
• User avatar icon in lock screen looks ugly and extremely low resolution
• Bring the option "lock screen on login" back! I am used to autologin + lock screen for single user machines
• Dolphin: when I add a comment, the text box is not selected by default, and I have to select it with the mouse before typing
• Videos for the example desktop effects do not work, and occasionally crash systemsettings
• I cannot make the top screen menu work as in KDE4, despite having appmenu-* installed
• If I set the application launcher to appear at a click on the desktop, there is a big (~1s) delay between the click and the popping up of the menu
• The default font is too thin on my 1280x800 13.3" screen. Bold font is perfect, I like it
• Autohide panels do not show and hide gracefully, but they simply blink in and out
• Terrible looks of several Dolphin and filebrowser-related parts (scrollbars, arrows etc.). This only happens on Arch as Kubuntu's packages seem to be patched against this
• In wobbly windows with a dark color scheme, a white pixel border line can be seen when moving windows
• Some icons seem to be missing (e.g. in Dolphin, Compact, Sort...)
• Wishlist: add the ability to add spacers in Dolphin's Places tab
• Energy information should not show rcu_preempt as taking up most of the CPU
• Energy information does not seem to gather battery Manufacturer correctly (upower -d does)
• In Energy Information, the "Battery" list should be a superset of the "manufacturer" and "Environment" titles
• Energy information graphs should have vertical tick lines with time axis
• Right-hand side of Energy information (incl. reload button) is partly overlapped by scrollbar
• Energy information should also report load averages and frequency statistics, like powertop
• The energy information program starts with a very small window
• Changing desktop theme to breeze dark seems to destroy the look of the panel (icons change color, panel doesn't)
• In desktop grid, a rectangle in the lower-right corner of each desktop is rendered incorrectly
• When you "show desktop", a phantom of open windows remain in the corners (may be a feature and not a bug)
• When using the Breeze dark color scheme and Breeze style, overlapping windows have seemingly no border and merge one into the other seamlessly, which is not ideal. This does not happen for white themes as the window shadow serves to separate the upper fromthe lower window
• Clicking on the KTorrent icon in the tray often does nothing or has huge delay

Again, I'll report these bugs and feature requests as soon as possible. In the meantime if some of these bugs are known please LMK.

Overall the Plasma experience has been positive and I'm satisfied. Well done!