IMHO RC2 is already high quality.
Many people (including the developers, and I) use SVN trunk. In terms of quality trunk usually is the least stable (has the highest likelihood of bugs). Yet it works amazingly well.
The Release candidates are from what is called a "fixes branch".
The fixes branch for all 1.2 RC and releases was created on 15 Sept 2013. So about 4 month back.
Since then this branch was used to improve stability. That is 4 month of fixes.
There are currently very few known regressions (thinks that used to work in previous versions, but no longer work) left. (One was regarding visual form inheritance).
As for SynEdit:
It is used in the IDE, if it had a regression, it would most likely have been reported by now. It also passes its testsuite.
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Otherwise it is impossible to say, what to expect. New bugs can happen everywhere (in the IDE, the LCL, any component, anything...) But overall, there where many 100 bugs fixed, since 1.0.x. So it is likely to either level out or be better.
Examples for issues can be found on mantis(bug tracker), or check what we already have fixed:
For 1.2 RC:
http://wiki.lazarus.freepascal.org/Lazarus_1.2_fixes_branch For 1.0.x
http://wiki.lazarus.freepascal.org/Lazarus_1.0_fixes_branchThe release page for 1.2.0 doesn't list any change that could affect what I'm doing, but you know, maybe the page got outdated (after all, it says nothing about SynEdit).
It is usually well maintained, but that said we are all human, and oversight is possible.
As for SynEdit, I do not recall anything which I believe to break compatibility.