Before 1947 the oldest complete Hebrew witnesses to Isaiah were medieval: the Cairo Codex of the Prophets, the Aleppo Codex of the tenth century, the Leningrad Codex of 1008 CE. The scroll pushed the evidence back about a thousand years at a stroke, and the question everyone asked was whether the medieval text would survive the comparison.
It largely did. The consonantal text of 1QIsa-a corresponds closely to the Masoretic Text, and the differences, though numerous, are overwhelmingly small. That result, that a text copied by hand for a millennium had been transmitted with this degree of stability, is the single most important finding from the scroll, and it should be stated without exaggeration in either direction.
The caveats are real and interesting. The scroll's orthography is markedly fuller than the Masoretic, using vowel letters far more freely, with distinctive spellings of pronouns and common words, so that the two look different on the page before any variant is counted. There are hundreds of differences of spelling, grammatical form and word order, occasional additions and omissions, and a scattering of readings that make better or worse sense than the Masoretic.
A minority of variants agree with the Septuagint against the Masoretic Text, which matters because it shows the Greek translators were sometimes working from a Hebrew text that genuinely differed rather than translating loosely. The best-known instance is at the end of Isaiah 53:11, where the scroll and the Greek both have the servant seeing light, a word absent from the Masoretic Text and now printed in many modern translations. A second, less complete Isaiah scroll from the same cave, 1QIsa-b, stands closer to the Masoretic Text than the great scroll does, so the caves attest more than one textual profile.