We cannot give one set answer because the matter depends on the individual person’s abilities and circumstances. Rather, we will explore the basic sources and logic, which will help enable one to apply the halachot wisely.
The gemara[1] deals with an apparent contradiction. One baraita says that if a klaf is missing some text, one can recite the missing parts by heart. Another baraita says that an illegible megilla is pasul, in which case using it is like reading by heart, which is invalid.[2] The gemara resolves the contradiction by stating that it depends on whether the majority of the klaf is proper or problematic. Accordingly, we can conclude that reading a minority of Megillat Esther without a kosher klaf is valid. However, the Shulchan Aruch[3] says that this is true only b’di’eved (i.e., one who has already read a minority from a non-kosher megilla need not repeat it or if one has only a megilla with just the majority written properly, he may use it[4]).
One who listens to the entire reading from a ba’al korei using a kosher megilla with the requisite intent and concentration fulfills the mitzva.[5] What happens if the listener misses a minority portion of the reading (either due to his lack of attention or because the ba’al korei read it incorrectly or could not be heard)?[6] If he is reading along correctly from his own klaf the entire time, there is no problem. If he had just been listening and following along visually, can he start reading from where he missed until he catches up with the ba’al korei? Does all of the reading need to be done by one reader? The Rama[7] says that since many in the congregation do not use kosher megillot, the ba’al korei must repeat the four p’sukim recited by the entire congregation, since their reading is like reciting by heart. The implication is that this would be unnecessary for those with a proper klaf, as the ba’al korei’s recitation of most of the Megilla along with the congregation’s reading of the four p’sukim would then combine for a proper reading. Thus, having more than one reader is acceptable. Actually, even one who is following with a printed megilla can also make up that which was missed by reciting that text on his own,[8] but since this is valid only b’di’eved (see above), it should be avoided.
Before we apply our findings, we will mention one more issue. The Shulchan Aruch[9] is concerned that one who reads along with the ba’al korei may cause others or even himself to concentrate on his reading at the expense of the ba’al korei’s, at least for some words. This is problematic if one is not reading from a kosher megilla.
The Pri Megadim[10] recommends reading along from a kosher megilla because it is often so noisy that one will surely miss some words. Although one can listen and then read aloud only when necessary, he apparently reasons that it is hard to identify the problem and react in time. Making up for missed words without a klaf would be valid only b’di’eved, and only if one concentrated on the ba’al korei’s reading most of the time.
On the other hand, there are disadvantages to following with a klaf. Many people will make serious mistakes in their own readings, and when focused on that, they may not sufficiently follow the ba’al korei’s proficient reading. There is also a fascinating machloket Acharonim[11] regarding whether one who reads for himself from a klaf has the preferred status of participating with a minyan.
We suggest as follows. If your shul is quiet (enough) that you can concentrate on the ba’al korei, read only as a makeup when you or the ba’al korei misses something. If your laining skills are good, it certainly pays to do so from a kosher megilla. If you will likely not hear a lot, read along the whole text (very quietly[12]), preferably from a kosher klaf. If the quality of your reading is mediocre or less, follow a printed megilla with vowels and read only to make up the parts that you miss.