What are the various opinions regarding the efficacy of the magician's in the Exodus story?
As you've probably heard, some indicate that they had real power; others that it was all sleight-of-hand. I believe Maimonides (Rambam) among others have the latter view, with the former view more traditional based on the literal reading (and various Aggada statements as well).
This is still debated by later commentaries as well.
Rambam's son R. Abraham, writes in his Torah commentary (Exodus 7: 11) in the name of R. Saadya Gaon (9th-10th century) that when the Torah states that "they too did so with magic" it means that they tried to do it.
He brings evidence that the term "they too did..." does not mean that they succeeded, from the later verse (8: 14) that states that "the magicians did so with their charms, to bring forth lice, but were unable".
וַיַּעֲשׂוּ־כֵ֨ן הַחַרְטֻמִּ֧ים בְּלָטֵיהֶ֛ם לְהוֹצִ֥יא אֶת־הַכִּנִּ֖ים וְלֹ֣א יָכֹ֑לוּ
Evidently the term "they did so" does not mean that they succeeded.
In R. Abraham's words:
ביאר ר׳ סעדיה ז״ל שטעמו רצו לעשות והביא עדות ממאמר ויעשו כן החרטמים בלטיהם להוציא את הכנים ולא יכלו
Rav Sa'adyah OBM explained that the intent is that they tried to do [magic], and he brought evidence from the verse: "the magicians did so with their charms, to bring forth lice, but were unable".
Similarly, Midrash Sekhel Tov (ed. Buber) states (Parashat Va'era 8:2) that when the hartumim conjured frogs, that this was a mere illusion:
ויעשו כן החרטומים בלטיהם. באחיזת עינים
Similarly, the Midrash Lekah Tov (Va'era 7:2) states that all the hartumim's acts were based on illusion:
כי מעשה חרטומי מצרים הכל באחיזת עינים היה
This is stated by Malbim as well (Exodus 8:3).
This is consistent with Rambam's position that in Hil. Avodah Zarah (11:17-8) that magic is ineffective and that anyone who believes in it is a fool.
However, there are numerous sources that assume that the sorcerers in Egypt could do actual magic.
Sanhedrin (67b) cites the opinion of R. Hiyya bar Abba that the Egyptians utilised demons and sorcery:
אמר רבי אייבו בר נגרי אמר רבי חייא בר אבא בלטיהם אלו מעשה שדים בלהטיהם אלו מעשה כשפים
This indicates that they did not merely perform slight of hand.
Additionally, the Zohar is quote here (although I cannot find it in the original), that the sorcerers, did not, merely use slight of hand.
Similarly, Rashi (8:14) explains that they were unable to create lice, since the power of the demons does not extend to things smaller than a grain of barley. This clearly indicates tat he understands that they did have actual power, and were successfully able to replicate the other miracles, such as turning staves into snakes (Exod. 7:11).