When I use InstaWallet.org, do I get my own Bitcoin address?
That address only receives deposits to your account. Otherwise, they'd have no idea what account to credit when funds wound up in that address.

But you can't use blockexplorer to monitor your balance, only to monitor the total amount of deposits you've made. InstaWallet will remove funds from that account as they wish to aggregate your funds with other deposits and service withdrawals.

Also, when you make a payment, even if InstaWallet uses your coins to make the payment, the "change" will be sent to a different address. For example, if you deposit 20 bitcoins and then withdraw 10, even if InstaWallet did use your 20 bitcoins to fund the withdrawal, the remaining 10 bitcoins would go to a different bitcoin address even though it's still in your InstaWallet account.

If, say, you're expecting someone to send you some Bitcoins, you could use blockexplorer to see if they've made that payment. But the balance you see won't really mean anything, it's just some number equal to or less than the total amount you've ever deposited through that address.