There are also a few articles in the critical literature (aside from the many commentaries that weigh in on the subject):
Grelot, Pierre. 1967. "Aujourd'hui tu seras avec moi dans le paradis, Luc 23:43." Revue biblique 74, 194-214.
Hong, Joseph. 1995. "Understanding and Translating 'Today' in Luke 23.43." Bible Translator 46:4, 408-417.
Kellermann, Ulrich. 1996. "Elia als Seelenführer der Verstorbenen oder Elia-Typologie in Lk 23,43 'Heute wirst du mit mir im Paradies sein' ". Biblische Notizen 83, 35-53.
I agree with the general consensus that parses the sentence as "Truly I say to you, Today you will be with me in Paradise". This has the support of the overall syntactic pattern of the AMEN formula, the immediate context in which sémeron "today" (= the present) is the counterpoint of the future-oriented mnésthéti "remember" (= after a lapse of time) and the indefinite hotan "whenever" (i.e. not "whenever" but "today") in v. 42, the broader NT context in which "being with the Lord" after death refers to an immediate dwelling with Christ in heaven (cf. 2 Corinthians 5:1-9, Philippians 1:21-24; compare 1 Thessalonians 4:17), and the overall eschatology of the intermediate state in Jewish-Christian literature and in Luke itself, which construes the pre-resurrected faithful dead in heaven or a state of blessedness with the patriarchs (cf. Luke 16:19-31, Wisdom 3:1-4, Apocalypse of Moses 37:5, 4 Maccabees 13:13-17, Testament of Abraham 20:14, Josephus, Bellum Judaicum, 3.374, Revelation 6:9, etc.), and which locates "Paradise" in heaven (cf. 2 Corinthians 5:1-9, 2 Enoch 8:1-3, 3 Baruch 4:6-8), see Apocalypse of Moses 37:5 for both concepts: "Lift him up into Paradise and into third heaven, and leave him there until that fearful day of my reckoning [i.e. when the resurrection for judgment occurs], which I will make in the world". In light of apocalyptic resurrection eschatology, the thief assumed that he would remain in Sheol/Hades until the resurrection for judgment (cf. Luke 16), whereas Jesus assures him that on account of his faith he will actually join him in the intermediate state in heavenly Paradise. In other words, Jesus enjoys a post-mortem intermediate state similar to that in Josephus or Revelation, but experiences bodily resurrection shortly thereafter, after which he ascends bodily to his glory. Compare Luke 23:46 in which Jesus commits his spirit into the hands of his Father and Gospel of Peter 5:19 in which he is "taken up" immediately from the cross, to then be resurrected shortly afterward (10:39-42). There appears to be no descensus ad inferos tradition in Luke; the references to Hades in Acts 2:26-31 are explicitly oriented to the body and its decay which was pre-empted by the resurrection, tho such a visit in the spirit is not ruled out if it is construed as related to the resurrection of the body.