I have a copy of Viewpoint and for some things it's very good (primarily correcting wide angle images with people at the periphery), but LR does the vast majority of what I need, and there's no need for generating additional TIFFs or PSDs for Viewpoint to operate on. If you haven't fully explored the capabilities in LR5, I'd spend some time there first. It's much easier and quicker than jumping to Viewpoint.