I have to put my vote in for the Hogue Beavertail grip. When I first got it I thought this thing is never going to go on, it's too small. It does take a little effort, and placing it in hot water for a couple of minutes, but once you get the knack of installing it, it's well worth it.
I wanted something that looked good, and something that gave me a little more to grab on with the Shield. I've seen too many pictures of Shields with Talons that just looked bad because of the seam where they overlap the back of the grip. I've also heard of way too many people saying that the Talons eventually start pealing because of that seam.
It does not slide down once installed. In fact since putting it on my Shield I have shot quite a few rounds and it hasn't moved at all, and it does not interfere with the magazine at all. It adds a little needed girth to the narrow grip of the Shield and it looks like it was something that the Shield came with from the factory. I can't say enough good about the Hogue Beavertail grip.