depends on position. early position on a table with aggressive players I'd fold it. early position on a table with nits I'd probably raise it. middle position pretty much same as early. late position: usually call might raise with it sometimes (particularly if I am the first one in the pot).
On the flop I'd be eager to fold if I am heavily raised with an Ace on the flop because I am likely beat out due to my weak kicker.
I will go into this basically looking to hit two pair or straight draw on the flop (assuming x=5 or less). If I don't hit this goal then I will see what other players do before I act (if I am late position then I will know what most players have done already so I can probably bet right there to steal the pot...if I am early then I will check to see what opponents do). If there were no bets I would probably try to steal the pot with half-pot bet. I wouldn't go too far with this steal attempt...so if I am reraised I would fold unless I have other tells
on my re-raising opponent.
Ax is not completely worthless but you have to be very careful with it. Hitting your A on the flop should make you more wary and eager to fold than optimistic and ready to bet; you must consider the fact that other people who are still in the pot may have also hit that A and their kicker is probably higher than yours.