pre-print

There's a very negative overview of it from a Czech physicist, Luboš Motl, but that was really the only negative reaction I found. Most of the others seem to be either neutral - much like yours, not entirely grasping the connection - or positive - though many of those seem to be people who Lisi drew his inspiration from in the first place, like Lee Smolin.

Some of the secondary commentary I came across suggest that Motl's response is more an indication of his attachment to string theory than a serious critique of Lisi's work.

I don't pretend to understand it either, but from what I have been able to follow, the connection is that the kinds of particle interactions described by SM (the kinds of things you can represent in Feynman diagrams, for instance) form an algebra, whose corresponding Lie group is E8. The group identifies the symmetries, which correspond to Noether-conserved quantities in the interactions.

Also, there's a semi-technical explanation of it on New Scientist.