Hunting for errata has become something of a sport, and the latest
printings contain more than 1000 corrections. The "official" list
of errata is available for free here: http://www.feynmanlectures.info/flp_errata.html
Only a handful of the corrections deal with physics concepts; the rest
are exceedingly minor typos. IMHO if you already own older editions
it is not worth replacing them, and it's not worth penciling in more
than a handful of the corrections. Open the errata-lists and search
for the word "wrong". As for the typos, it suffices to be generally
aware that they exist.
The last time I checked, the paperback editions were more up-to-date,
containing more of the corrections. That's because they were more
recently printed, in contrast to the hardback editions where were
apparently printed a while ago in huge numbers and will not be
reprinted until the stock is exhausted.
Also, the last time I checked, buying the three paperback volumes was
not cheaper than buying a boxed set of the three hardbound volumes ...
which actually makes some sense, given that it only costs a buck or
two to put hardcovers on a book.
By way of contrast, note that the so-called "Definitive Edition" was
far from definitive. It is older than the "New Millennium" edition
and in most ways less desirable.
This volume contains the three long-lost Feynman lectures on problem-
solving techniques, plus the Leighton/Vogt exercises ... many of which
are quite thought-provoking.