None of these translations are in a "modern style," or perhaps I should say that they aren't dumbed down as most popular presentations tend to be. They are in the language of a brilliant professor of linguistics who was hesitant to replace a perfectly good medieval word with an inaccurate, newer one. Happily, there's a decent glossary in the back for those of us who are a bit more uneducated.
The big draw here is probably Sir Gawain and the Green Knight, being an Authurian tale that is usually given a quick gloss-over along with Chaucer in high school. Joseph Campbell also tended to cover it in his many books on heroic mythology. So, no surprises here as to the story, which leaves one free to just enjoy the language.
Tolkien does his best to adhere to the original alliteration and verse scheme, here and in the other two poems. There's a technical essay at the end of the book that explains his method versus the Pearl author's, but the lines themselves are self-evident in their structure. There's a bit of archaic reversal of our modern noun-verb-adjective word order whenever the syllablic accent count demands, but most of the challenge comes from the usage of technical terms for armor and such.
Being over-familiar with the story, the real surprise for me upon reading this close translation was the inclusion of a long passage giving the symbolic interpretations of the pentacle on Gaiwan's shield. The points are compared to the knight's five faultless senses, his unfailing five fingers, and the five wound of Christ, among other things. Usually in these stories, we're expected to understand the allegorical mapping of events onto reality in an instinctive fashion, as above so below and all that. To be given such a precise amount of information reveals how important this particular symbol seems to be for the author, or perhaps for the sect that he belongs to.
The Pearl is the more difficult poem, and the more satisfying to work through. At it's heart is a dream-time dialectic between a father and his deceased, innocent pearl of a daughter about Jesus' parable of the workers in the vineyard, as given in Mark. The father (and narrator), even through his grief, expresses the worldly view that earning one's way is the proper and moral structure of things. His missed daughter has to point out that if that were true, then she would not have "earned" the right to live in the city of God. Grace wins out over judgement.
Before he wakes up, the narrator also give a long description of New Jerusalem, after Revelation, using lots of technical terms for precious jewels. There are probably all sorts of extra meanings for each level of the city given, defined by different types of gems used; unlike the pentacle passage however, we're not given any kind of decoding. Another type of communication lost to us moderns.
The Sir Orfeo poem is an odd transposition of Orpheus to the fairy world of Breton. It's a simple story, and a simple rhyming scheme. It's also not very satisfying to read after conquering the complexities of Gawain and The Pearl. I found it interesting that Faerie is presented as the land of the dead rather than Hel, and that Orfeo has a bit of a Ulysses-in-disguise homecoming, but there isn't much else in the way of story to recommend this over Virgil's older version. I imagine Christopher Tolkien meant it to be a sort of relieving bit of fluff to fill out the book, but it just comes off as something his father worked on in his spare time the way you or I would do a crossword puzzle.