It looks like Consumer Reports did a pretty good converter box comparison between the Zenith DTT900, Magnavox TB100MW9, and the Insignia NS-DXA1.

All three boxes were given thumbs up for ease of installation and use, as well each one of them having a program guide (tv channel guide) feature. The Magnavox program guide is a deal breaker in my opinion since it'll only display TV shows on the current channel, unlike the DTT900 and NS-DXA1 program guides which will show current and next programs on all channels.

The comparison results were pretty close with each converter having far superior performance than what an older analog television would need (which is a good thing).

Side Note: Consumer Reports mentioned that they purchase all of their test converter boxes from retail stores just like consumers, and they have also experienced limited availability of the boxes. Hopefully we'll see some more supply to fill the extra demand soon!