By rarities boxset do you mean the two Genesis Archive boxes?
They're both excellent, the only thing wrong with them is the sightly weird tracklisting. On the Collins years box, two great rarities are omitted in favor of dumb remixes, and the Gabriel box sequences everything chronologically backwards. But those are minor complaints. The sound is fantastic throughout, the rare songs are terrific for the most part, and they give insight into the development of the band. And you get live versions of just about everything that wasn't already on a live album. Highly recommended.