- Can I enable logging when I use(from same server) and disable it
for general public ?
Sure, but you'll have to implement it yourself. Using an IP Address is a common approach. Using a login username / ID is another approach.
- how much of over head will logging cause, If I am liberal in
putting log statements ?
It depends what you're logging, how you're logging it, how often you log it. Taking full memory snapshots of all components is going to be a lot more resource intensive than just keeping track of the time that a user logs in.
- Are there any other disadvantages of using logging in Flex
Application ?
It will take time and resources to build a logging system; as well as time and resources to review and analyze the results.
Your question is so open ended there is no answer. If I were to ask you "How do I program?" or "What programming language should I use?" would you know where to start?