A lot of large companies have ripped off or been accused of ripping off other companies and made millions if not billions e.g. Xero and Apple UI, Microsoft & Apple UI, Facebook & MySpace, MySpace & Friendster, etc. It is common but this doesn't exactly speak to the ethics, which is what you are asking.

I am a similar situation as you also working on a similar product. Here is what I would ask myself:

- Are you intending to copy them?

- Is their product particularly novel?

- What are you doing differently? Price, UX, features I require, integrating into different API's, etc.

I would highly focus in on differentiation, what makes you different from them? Are they too expensive, so you are disrupting their market with a comparable lower cost option. This is how a free market is supposed to work in a capitalist society.

Edit: Formatting