The first series pulls off an astonishing twist ending that I won't be spoiling here. The second part of this review will discuss the twist, and the implications, in some detail.
On the surface Magic Knight Rayearth is surprisingly unsubtle for a CLAMP production. Let's start with the characters who are:
a) Ryuuzaki Umi: attends a prestigious private school with a blue uniform, fences, and has a name that can be read as "Great Dragon of the Ocean".
b) Hououji Fuu: attends an elite school for girls with a green uniform, an archer, and has a name that can be read as "Green Wind".
c) Shidou Hikaru: attends a public school with a red uniform, heir to a kendo dojo, and has a name that can be read as "Lion of Light".
Can you guess which character will be the Magic Knight of Fire? Of Water? Of Wind?
The setup to the story is about on the level of an RPG where everyone meets in a Tavern because the GM couldn't think of anything better . In this case all three schools have scheduled school trips to Tokyo Tower  at the same time and the young ladies accidentally cross paths . In that moment they are summoned by Princess Emeraude to Cephiro to become the Magic Knights.
On arrival they are met by the sorcerer Clef who provides them with minimal briefing and equipment. The key point being that they can't go home until they become Magic Knights, save Cephiro from a looming disaster, and grant the wish of Princess Emeraude.
Cue the quest for plot coupons , and one that Fuu is more than willing to lampshade on several occasions.
It is very easy to let the fabulous opening credits, and the sheer energy and enthusiasm of Hikaru, carry you through most of the first series without looking beyond the surface.
However there is a lot more going on.
To start with I have to wonder just how familiar CLAMP are with the classic Arthurian mythos. One of the classic situations in the Arthurian mythos is a test of virtue, often to end an evil custom by fulfilling it in a just manner. There are times when Magic Knight Rayearth reeks of this, and it certainly leapt out at me a lot more on the most recent rewatch.
There is also a definite rhythm to the episodes that tests each of the ladies in turn, before testing the group. Whether this results in character development as opposed to simply becoming more powerful is debatable, but it certainly results in a tightly knit trio that can overcome anything thrown at them.
The other aspect of the testing is that it ensures that the ladies earn their powers (or create them for themselves), rather than being gifted with them. The former carries a strong coming of age and/or empowerment message with it, the latter is more damaging in the long term. 
The progressively darker tone of the series as the world of Cephiro begins to crumble around them is well handled. This adds a sense of urgency to the quest and is done quite subtly to begin with but accelerates towards the end of the series. One nice touch is the titles for the individual DVDs in the English language release: Daybreak, Sunrise, Noon, Twilight, Midnight.
Finally, although Hikaru is unquestionably the primary heroine , a lot of thought went into the fighting and magical styles of all three main characters. They are surprisingly well balanced in a number of ways, and this becomes a plot point in the second season.
There are a number of weaknesses.
The show sometimes feels a bit padded , and the recaps in every episode can get a bit wearing. The comedy episodes with the SD character designs are amusing up to a point, but again can wear out their welcome sometimes.
The rematches with Alcyone and Ascot in the early episodes can inspire a certain amount of snark. At one point I was tweeting a score board after each episode (e.g. Magic Knights 5, Alcyone 0). On the other hand it did help to keep the number of characters down to a manageable level.
Finally the twist ending can generate some cognitive dissonance that I'll be talking about in a separate post.
Magic Knight Rayearth remains one of my favourites. It is not perfect but there is a lot to recommend here.
 Yes I've done that. I think most GMs have at one point or another. Cliché: another word for inevitable.
 Tokyo Tower is the mystical and magical heart of Tokyo according to CLAMP and several others. But you knew that already. :)
 And if you believe that I have a bridge to sell you.
 Scroll down to Part 4 of the Turkey City Lexicon.
 This was one of the tropes Steveg and I discussed at some length in our Anime Archetypes panel in 2010.
 The title alone makes this clear - in a more balanced series it would have been Magic Knight Cephiro not Magic Knight Rayearth, although it is a spoiler to say why.
 Although not to the utterly egregious extent to which the second season was padded out.