(from the book Baseball Prospectus, by Clay Davenport)
"EQA (Equivalent Average)...evaluates a player's statistical line to estimate how many runs the performance was worth to the league as a whole. In the case of a very bad hitter, the performance might even be worth negative runs. This is the player's Equivalent Runs, or EQR. EQA is basically EQR per out, although there's some extra math thrown in (to make it look like a batting average)."
To determine if someone's EQA is good or bad, rate it like you would a normal batting average. ".225 stinks, .300 is a recognized achievement, .350 puts you at or near the top of the league, and .400 is almost mythic."
As a comparison, Mike Piazza had a batting average of .303 in 1999, and an EQA of .301.