Although my central mottos are: "Never change a running system." and "Newer isn't always better!" I too upgraded to 5.8.11 a few months ago. Until this time I was still using 5.8.7.0.1 (the last fw that could be directly downgraded to the factory build, just in case ...).
The user fw v5.8.11 in my eyes is very stable. And though I'm using my Kindle only as a good hardware platform (never ever connected it to Amazon, thus my only books in Amazon formats were the Users Manual and the dictionaries which came out of the box), I've decided to convert some technical and otherwise specialized books in mobi and ePub with lots of formatting etc. via Calibre to the new AZW3 format, and read them using the native Amazon firmware instead of the otherwise preferred CoolReader. And - BINGO! - the new boldness options are becoming more and more a real benefit for me in these cases! (Especially in connection with an appropriate FONT_RAMP file.) So 5.8.11 is really worth a try.
