an index on a column that has only two values will likely not be used at all, because it isn't selective enough
may i ask why you thought it necessary to make that change?
from a performance point of view, it's a wash, but from a portability point of view, you've locked yourself into a proprietary mysql format that is not supported by other database systems
also, there is a ~real~ big problem with enums that happen to have string values like '0' and '1', it is too easy to make a coding error and get the wrong value
how easy is it, do you think, to write a php script which is sometimes just a wee bit lax in distinguishing between 0 and '0'? the difference here will kill ya