It's not a theory that makes quantitative predictions, it's more a blueprint for a future theory, and the critics would say that the blueprint is hopelessly flawed - that no such theory is mathematically possible.
The larger theoretical context of Lisi's work is the attempt to describe 4D gravity as a gauge theory, the viability of which is the central dispute between string and loop theories of quantum gravity. Lisi's theory is a "GraviGUT" theory which then adds to this problematic foundation, even wilder hopes about getting fermions from "BRST ghosts", and about finding loopholes in theorems which say that even then, you couldn't get the necessary three generations of them, out of a single E8 gauge field.
Incidentally, there are various ways to get three generations of particles "from E8" in string theory, so perhaps those should be regarded as the real "E8 theories".
I never understood how Lisi's E8 got around the Weinberg-Witten no-go. Was there some reason it didn't apply?
There are a great many ideas which don't have enough carefully-measured evidence to be sufficiently confirmed as scientific fact and accepted as such by the scientific community (a recent joke was "While the Higgs Boson has not been discovered yet, its mass is 125 GeV"), but don't have enough carefully-measured evidence to be ruled out yet, either. Do any of the tools of the LW community help narrow down which ones are more worthy of consideration than others?
Eg:
* Cryonics as an arguably reasonable bet for its cost: proto-science
* Cryonics as a surefire way to achieve immortality: nigh-certainly pseudoscience (unless it's the method by which your Everett Immortality keeps you alive)
* Using math to demonstrate that taking classical physics and adding determinism results in MWI-style quantum physics: proto-science.
* Using math to demonstrate that quantum physics proves Christianity is true, from a certain point of view: pseudo-science
* Tubulin might self-organize into microtubules capable of computation on a sub-neuron scale: Possibly proto-science
* Tubulin architecture is 'quantum' in nature and that is what gives rise to consciousness: Probably pseudo-science
* 'Quantum consciousness' means anything is possible: Downright silly
* The E8 Lie group can provide a system for organizing the properties of subatomic particles: Proto-science, perhaps
* Heim theory is useful for predicting particle masses: Pseudo-science, probabilistically
* Using the Bullet Cluster to claim that dark matter is a better theory than Modified Newtonian Dynamics: proto-science
* Claiming that dark matter is made of 'anapoles': Proto-science, perchance
* Suggesting that dark matter is actually gravitational leakage from MWI 'parallel universes': You tell me. (But if it's true, then since I can't seem to find any previous serious discussion of this idea, I get to name part of it after myself, right? :) )
These may not be the best examples, but they're the closest ones I can think of to the boundary. If you know of any better ones, feel free to comment with them.