This is something I often debate as well. On one hand, it allows you to go to market faster. On the other, your business is perhaps less defensible. That said,

This is something I often debate as well. On one hand, it allows you to go to market faster. On the other, your business is perhaps less defensible. That said, even if you went out built your own solution from the ground up who's to say someone else wouldn't come along and just piggyback the API you're using now? As long as you're aware of the potential business risks and are taking at least some steps to mitigate it (or have plans to), you might as well keep adding as much value as possible to further position yourself in whatever market you're playing in. Focus on the value add, and if you start to reach a scale where pricing becomes prohibitive, great! Now build your own. If their API changes, well, that's a bit harder to handle. The second you start to see any level of serious traction you'll probably want to either a) lock down a long term contract or b) build a replacement that frees you from this dependancy.