There is 1 more big positive - you can maintain your playfield in mint condition if you install a protector on a new game - zero dimples. Really helps out on games with ball drops and/or tendency for airballs or magnets. Worth a $150 to me.
It is true that dirt gets under them, but no "grinding" - the protector shouldn't be sliding around if installed correctly. You can take them off every couple of years if you want to clean them. You can also just cut them off with snips in about 5 minutes if you want to remove them permanently to, say, sell your game.
As far as speed difference, ask 3 people and you'll probably get 3 different answers. I have them installed on everything from old EMs to modern games, and I personally think the speed difference is negligible. Hell, some people even say the ball travels faster.
In the end, YOU are either going to like them or not, but nobody else can tell you if YOU are going to like them or not. Just like I can't tell you what your favorite color is, even though I know it's green Try one and see for yourself.