My grandchildren like the others grandparents more than they like us. Really it comes down to two simple things:

1. Their other grandparents buy them all kinds of stuff we can't afford.
2. They can get away with anything with the other grandparents.

Personally, it doesn't bother me. I've been a step parent for over 13 years and I'm accustomed to being considered the worst person on the planet. But when it comes right down to it, my daughter (step-daughter) would rather bring her kids over to us because she and her husband know that we will respect their wishes as far as gifts and discipline go. We give the grandkids more what they need (to the best of our knowledge and ability) as opposed to what they want and we're fine with that.