To what degree do the Dead Sea Scrolls of the book of Isaiah match our version of the book of Isaiah?
The text of 1QIsaᵃ (The so-called "Great Isiah Scroll") is very close to the masoretic text (M). Most of the differences are orthographic. 1QIsaᵃ usually is fuller, employing more matres lectionis (e.g. כי in M vs. כיא in 1QIsaᵃ in verse 1:2). Other small differences exist due to pronunciation (e.g. עוזיהו in M and עיזיה in 1QIsaᵃ in verse 1:1).
Letters or words were occasionally added to 1QIsaᵃ supralinearly as a correction, or deleted. Usually the correction brings the spelling closer to that of the masoretic text (e.g. the insertion of the weak ע in the word יש[ע]יהו in 1:1).
There are also times where 1QIsaᵃ agrees with the Old Greek (LXX) or other versions over M. For example, in Isaiah 36:11, we have העם in M but האנשים in 1QIsaᵃ, corresponding to the form in the LXX (ἀνθρώπων).
For more on the phenomena see Emmanuel Tov's "Textual Criticism of the Hebrew Bible". For specific cases, see the critical editions published by the Hebrew University (page example) or the Biblia Hebraica series (Quinta is the newest and finally uses all the Dead Sea Scrolls, but the edition is currently in preparation).
For the degree of correlation of the Great Isaiah Scroll (I Qa) to the Masoretic text, Gleason Archer in A Survey of Old Testament Introduction (Chicago: Moody Press, 1994) states: “Even though the two copies of Isaiah discovered in Qumran Cave 1 near the Dead Sea in 1947 were a thousand years earlier than the oldest dated manuscript previously known (AD [sic] 980), they proved to be word for word identical with our standard Hebrew Bible in more than 95 percent of the text. The five percent of variation consisted chiefly of obvious slips of the pen and variations in spelling.” For more information and bibliography, see 'Great Isaiah Scroll' in Wikipedia (https://en.wikipedia.org/wiki/Isaiah_scroll); Donald W. Parry and Elisha Qimron, editors. The Great Isaiah Scroll (Isaa): A New Edition (Studies on the Texts of the Desert of Judah) (Brill Academic Publishers, 1999).
From what I have read the Qumran scrolls have about a 95% correaltion with our text and the remaining 5% is accounted for mainly by spelling variations that do not change the message.