When I was searching for an HDMI converter I specifically chose this one for the zoom feature:
http://www.ebay.com/itm/HDMI-to-CVBS...UAAOSwA4dWLd54
I have an Apple TV connected to it, and in turn the converter is connected to an RF modulator feeding into a mid-80s Zenith TV. Picture quality is excellent, and the unit itself seems to be well made.
Most of my viewing consists of Netflix and YouTube videos. 90% of the time I have no issues with black bars on my screen, except for older YouTube videos with a different aspect ratio (black pillars) and the occasional movie in cinema format. Considering how well the setup works, I can live with the odd video that doesn't fill the screen.