I just got one and it's not as cheap as they may lead you to believe. Yes, you can lease a new Legacy for around $319 per month, but if you want to step up to the GT (or even the GT Ltd), depending on options, you're going to be looking at a MSRP of somewhere in excess of $31-$32K.
Coming in with little or no down, you'll be in the mid-$400's monthly, but you still have to add your local taxes to that. Bottom line, depending on model and option, for a 39 month lease at 15,000 miles per year, I would expect to be around the high $400 range TOTAL.
The only good news? Subaru's promotion will include standard maintenance. Could be worse! I jumped out of a lease on our '02 Xterra and had negative equity of $4,000 which we had to build into the new lease. Leasing can really suck, unless you can write off the payments/insurance/gas/etc through a business, which fortunately I can.
Good luck!