Ask HN: Where did you get your best product feedback during development?

Our startup Lucolo (http://www.lucolo.com) is creating a message exchange service for enterprise environments. It's an alternative to email and, as being such, we're trying trying to determine the best features to build and market for our launch but we'd love more feedback from our prospective users to help us hone in on this market and how they'd use Lucolo. We've created a Google form (http://goo.gl/forms/DbDhxD81rB) and have been sending it out to legal, government, and financial organizations via email (we've found contact titles and emails through sites like http://www.lead411.com) but I'm curious to hear what other folks have done to engage with prospective user groups to garner feedback to validate product-market fit.

Thanks in advance and I can't wait to hear the responses!