See above - I believe that it is just as constitutional as the Posse Comitatus Act. If Congress can limit what the president can do as commander-in-chief domestically it can also limit what he does internationally, right?
The Resolution gives to the president pretty broad latitude for employing force without congressional authorization. What it doesn't do, is give him a blank check where clearly defined American interests aren't at stake. I think all things considered, that's a pretty could compromise and an effective check on executive power.
Well, the only "effective checks" Congress has, with or without the War Powers Act, are the same: the power of the purse and the power of impeachment. Otherwise, they have no Constitutional power in this area.
As for Posse Comitatus, it would seem to me to have the same net effect (i.e. none).
If you have a President, such as the current one, who feels no obligation to obey or enforce the law or the Constitution or his oath and you have a Congress that is unwilling to use the only power it has, then you have a President without any effective checks at all.
The Constitution does not define what a Declaration of War is, how it shoud be worded or even if the words "Declare" and "War" need to be included -- it leaves that up to Congress. But whatever form it may take, or not take, the levers Congress has to control the war (or the President) remain the same.