If your main feature is googles search engine on y our website, could that be bad?

Not bad, but not particularly useful since people will sooner search Google from Google and not your site.

If you've used the Google custom search engine and set it up in a way so it only presents results from specific sites that you think a certain group of people will like then you're adding value on top of the generic Google search, which would be good.

It's not bad, but it's not good, either. Google won't send any people your way, because if all they can discover on your site is Google, well, those people already know about Google.

not bad...but probably not the best use of your time...move to to something that hasn't been done yet and good luck!