I got one of these and am absolutely thrilled as can be with it. It tells you the slope and where straight is. And of course, if you know the % grade of the slope, you have a really good idea of how hard to hit the putt.
Everyone should get one because now we can practice developing feel with our feet by ourselves. Amazing training aid.
PS- In case you were wondering what this is, it's a modified spirit level (or bubble level or builder's bubble, whatever term you like to use).
PS- I'm not soliciting nor do I work for them; I'm just trying to make it easier to order it if you want one. I just love this little doohickey.