Large trucks make this road a nightmare for the average motorist, especially because no single roadway is ever wider than three lanes. Also, deplorable pavement conditions east of NJ 24 (the original roadway section) not only knock that rating down from a 9, but contribute to traffic problems and accidents. The only saving grace is that the median west of I-287 has been the recent recipient of a concrete Jersey barrier; crossover accidents and fatalities plagued that stretch for years.
With the trucks, weekend traffic to and from Pennsylvania, suburban sprawl by the I-287 interchange area, and rush hour further east, the maximum rate of traffic flow for much of the highway tends to be around 60 MPH. Speeds are somewhat better between 287 and NJ 24 (the three highways form a triangle around north-central Jersey, so westbound and eastbound traffic get split off the legs).
East of the NJ Turnpike, I-78 becomes the Newark Bay Extension, which at four lanes to the Holland Tunnel usually has between a half-hour and an hour delay during normal travel hours.
Signage overall is quite good. Although the signage west of NJ 24 is all non-reflective button copy, destinations are all clear, and the signs are in generally good condition. East of 24, the local / express setup makes things a bit harder to follow, especially near US 1-9, I-95 and Newark Airport, where the proliferation of ramps and exits can confuse even a lifelong NJ motorist.
Finally, the highway, though quite beautiful through the Watchung Reservation and the gently rolling hills of western Jersey, suffers greatly from the wide concrete swath through the ugliest urban neighborhoods of the Newark area.
I-78 really is two different highways west and east of NJ 24. The western portion should probably score a good 20 points higher than the eastern (especially pavement, signing and aesthetics).