I totally love Cartwheel! It has great % discount and you can use in on top of any coupons, sales or promotions! The most common discount is 10-15% (which is already good!), but they often have 25-50% as well, which makes it a great deal! And now you don't have to use your facebook to register/log in, they just came up with email login! :)
Definitely give it a shot!