Consider how the solution might work for all client-facing employees. Not just SDRs / inside sales. This solution would be useful for anyone involved in retention, cross-sell or up-sell; Account Managers, consultants, etc.
OppSource solves the SDR toolset. It provides functionality that is not native to SFDC or other CRMs, but that is essential to maximize SDR productivity. We have seen the sales cycle accelerate, the SDRs happier, and our cost reduced (no SFDC seats required for SDRs using OppSource).
We are looking at rolling this out for other front line staff that aren't typically thought of as direct sales, Account Managers, CSRs, etc. This will increase the results in retention, upsell & cross sell.