I regret nothing ;) the speed and agility one gains from 'light' but imperfect solutions is far better than the effort required to do anything else.
Plain text is best of course, but binary formats are easier for dimwit colleagues to understand - and I get paid quicker that way too. We both have problems diff'ing between binary formats so most of the time they don't bother and neither do I - no loss really because most documents have a very limited lifetime (especially when you use LiveLink :). It's kind of a devil's pact - I promise to write documents in MS Word if you promise never to ask me to figure out the history of changes or keep older versions.
Email is the same story really- I never really searched through past email, but I did lose archives during sync operations and missed having access to email when at the office. Gmail wins this battle every time. Another devil's pact - I promise to reply to emails quickly wherever I am if you promise never to ask me to remember what we talked about.
It sounds a bit lackadaisical - and it is - because life's really like that - and in the real world of the idiots no one even backs up their computers - so we are already on to a winner! And in the real world it's always better to have an immediate if imperfect (but rosier) recall of past events than it is to say 'yeah, well let me just go and grep that for you' because not only does that sound dorky, but no one will thank you for remembering the truth (not if you want to be president that is :)
OK, so when it gets serious, i.e. when I start coding (because emails, documents, spreadsheets are just the pointless stuff that stops me from coding) then it's plain text, Python and Mercurial SCM every single time. No argument. Colleagues can't understand how Mercurial works? Then I tell them to find another job. Another devil's pact ... let me use my own tools and I will write good software for you.
Basically, aside from coding work, life's too short to worry about retracing your steps --- it's much easier (and rosier) to try and remember what you 'think' happened and go from there.
One past client had a rule that all email was auto-deleted after two months ... sound horrific, but boy did it stop all the 'you said' 'he said' arguments... (also, stopped any horrible litigation :)
The killer app in all these years, as I transition from thinking like a coder to thinking like a mild alcoholic, has been Freemind, which helps me organise my thoughts and tasks but is practically useless for keeping a history of changes (ok, so it does, but the whole 'history' keeping doesn't work in mind maps). The thing about Freemind is that keeping information in a mind map somehow etches that same information in your brain - so I can remember almost exactly what's in my Freemind project map.
So, in summary, drink vodka...