I love this feature, but unfortunately I've found that probably 30-40% of the websites that I supply with my email address will not accept it due to the "+" character. It seems like travel-related businesses are some of the worst offenders -- the vast majority of airlines and hotels seem to balk at the "+", which is frustrating, because "+travel" is one of my favorite filters.
Amusingly, this is almost certainly due to the legacy backend systems which still use a six-bit character set, and this set doesn't include helpful things such as lowercase characters or the + symbol.
