Here's what's important to me.
1. How well do they listen when it comes to helping you find a house? After narrowing it down a bit and getting to know you, they should be able to focus in on what YOU are looking for.
2. Availability - do they return phone calls promptly?
3. Service - Can they answer all your questions, provide you with qualified referrals, etc.
4. Willingness to work for you - they aren't doing you a favor by helping you buy/sell; they are working for you. Do you feel that way? You should.