For me baptism is an act of discipleship. Unlike some traditions that see baptism as an entry point into the Kingdom of God (and thus it can happen to a child), I would assert that entry or belonging to the Kingdom of God is more a dynamic of faith than a physical act. There is, of course, a physical act involved in baptism, and for that reason some Baptist Churches (not Hawera) still link it to "Membership", however in my thinking that act is more correctly understood as one of discipleship rather than belonging. It's an individual decision, rather than a parental or community decision. This decision is made as a response to the reality of faith in Jesus that has already occurred for that person, which of course usually happens within the context of a family and/or community.
I would consider baptism to be a step of obedience. Jesus said, "believe and be baptised" and so we follow the model that was established by Jesus and was the practice of the early church. Related to this idea is that in some way we are "following" Jesus when we respond in obedience, from our own conscience, rather than the decision of others.
It's also a public statement of belief - and quite an extreme statement at that. When you think about it, it's rather "different" for a teenager or adult to step into a body of water, fully clothed, and have someone guide them to full immersion and out again. However it is this very act and the symbolism attached to it (eg. dying to self, rising to new life, submission, cleansing etc.), that makes the statement so strong and clearly shows ones intention to follow Jesus for the rest of your life.
So that's what I believe, and with some small variations, what most Baptists have believed in the past. However we are living in a mixed up world! No longer are Baptist Churches full of Baptist people, rather we're a collection of Christian people from various backgrounds and different traditions. For this reason alone I would be hesitant to say that "believer's baptism is the only form of baptism". However, it is important that we don't simply give up on believer's baptism by immersion because some might think differently. As a Baptist Church we need to continue to promote "believer's baptism" and challenge people to this high calling of discipleship, obedience and public witness. We should see more of it, not less of it, even while accepting that some among us will have had quite a different experience and find God's hand and significance in that. What do you think?