What email marketing integrations have people had success with for NetSuite? Doing research on options and so far I've only found 3rd party integrations with NetSuite

Bronto would be the obvious one to look at, as Oracle own them.

Any experience with that? I've been investigating that one and Constant Contact with a 3rd party connector to NetSuite

No - we've had several conversations with them in the past, but are quite loyal to our existing (UK) provider at the moment. I suspect we'll have to consider them more seriously in future though, as they obviously understand Netsuite data far more than other providers.