I think it's important for the buyer/seller and real estate broker to understand one another well and have a good relationship. However, it's not just about whether or not you 'like' someone. Other factors that should be included in your decision are the broker's compentency, negotiation skills, area knowledge, education, experience, and assertiveness in representing your interests.
For instance, I would rather have the best attorney representing me in a case, rather than some attorney who I just happen to like the most but may not be the best at what they do...
I hope this helps.
You should definitely work with a realtor whom you feel most comfortable with and whom you feel best represents your needs.
If you have already listed with a realtor to sell your current home and are wondering if it makes sense to use the same realtor to purchase your new home; that is a question only you can answer. Some realtors offer a an incentive if you use them for both transactions, some do not.
If your listing agent is currently doing everything possible to represent your needs and to net the best possible price for you; likely that same agent would similarly do everything possible to represent your best interests when helping you purchase your next home. By using the same agent for both, your communication stream would be easier and cleaner (especially if you have a good rapport, with your current agent) and the potential for having both transactions coordinated and completed in a timely fashion is probably greater. Good agents are just that; they enjoy helping their clients and building lasting relationships. They work diligently to represent you, their client. They aren't in real estate to just "make it big" on one transaction or a series of transactions. They recognize the lion share of their business comes from repeat business and referrals. It's all about building lasting relationships.
What you want in a Realtor is an agent who is looking out for your best interests, not the other party's best interests.