It's easy to use the credit.. your debit card will still work up to that limit. Also, you get to see "Overdraft Interest" paid plastered on your account.
It's not meant to be used as a normal LOC, it's a buffer. If you were at any other bank or credit union, you'd be paying $35 per OD.
I wonder if they ever terminate accounts for excessive OD LOC usage. (I've had my account since they were ING Direct, but I haven't used the actual account, other then for depositing checks, since August of last year.. my credit union deposits my direct deposit 2 days early, so I use them!