the measurement wasn't accurate due to the somewhat crude nature of both measuring the angle of elevation and the distance between the two cities. it is rather accurate however, as it was the first measure of the circumference of the earth ever done... you could replicate this experiment today, it just requires good timing and a small amount of planning.
two other interesting things you could estimate from the results are the volume and surface area of the earth, all you have to do is assume that the earth is a sphere which gives a decent amount of error but still rather good for so long ago (not sure when Eratosthenes did this, but if it was after 225 BC he would have had the formulas proven by Archimedes and been able to calculate the surface area at least).
the surface area from the given radius is 487,009,391 sq km (real value 510,100,000 sq km so error of 4.5%)
volume from given radius is 1,055,394,856,531 cu km (real value 1,083,210,000,000 so error of 2.6%)