On the downside... no way would I use a tax preparation system provided by the IRS. It's bound to be a UX nightmare, like all government software, there's an obvious conflict of interest (the IRS has no reason to help find deductions and/or assist me in minimizing my tax bill), and I'd rather not give them any more visibility than I have to into my personal finances.
Update for downvoters: Woah! Sorry for casting aspersions at the Tax Man.