We previously were using 5.3.x cycle for a long time as bugs stopped us upgrading despite some useful new features, however 5.5.1 has a number of issues which make it look less than ready/fit for use, I hope they are fixed soon.
e.g.
Editor crashing (Actually freezing indefinitely - no message) on windows 10! (reported bug)
Editor freezing or becoming unresponsive for some time with no visible message/indication of what it is doing - (at the start of a build of a large project for instance).
GUID errors: “Could not extract GUID in text file projectSettings…”
Mac build on windows is just weird and buggy when selecting the target location - either a bug or poorly changed behaviour
Console messages which say: “This message has been deprecated and will be removed in a later version of Unity.”!
UI Text turns black - have to restart editor to fix
