Other entries in this genre such as Gasaraki or RahXephon twist already complicated mythologies making it difficult to work out what just happened. Xam'd avoids this by mostly inventing its own, simpler, mythology with significant shout outs to Nausicaa and, now that I think about it, Haibane Renmei.  The Nausicaa shout out is made explicit via one of the characters.
This is not to say that Xam'd is easy to follow, only that many of the complexities come from having to keep track of a too-large cast with too-many plot lines.
Most of the supporting cast is well handled, but you can tell the exact point when the numbers overwhelmed the scriptwriters . The solution was to dump a bunch on in an out of the way spot and then keep them off screen whilst they were repairing their ship .
At its core though Xam'd is a coming of age and romance story for high school students Akiyuki Takehara and Haru Nishimura. Incidentally their given names mean autumn and spring respectively.
The development of both characters, and of the partnership, is deftly handled. Haru can get a little distracted or confused when older characters manipulate her, but this is balanced by impressive determination once she works out what she needs to do.
The story of the Nausicaa equivalent Nakiami  is initially tightly interwoven with Akiyuki's but is not complicated by any hint of romantic triangle . Partly this is because Nakiami's arc is one of emotional development .
Partly it is because even though Akiyuki is initially dependent on Nakiami for his survival he remains faithful to Haru throughout .
I must say that it is nice to see a fairly simple romance in an anime series for a change: he wants her, she wants him, heaven help anyone who gets in their way once they work up the courage to finally say so.
The finale is well constructed and quite effective, although the impact is somewhat lessened by the epilogue containing some unexplained "I Got Better"s.
Technically Xam'd looks, and sounds, fabulous. The fact that I was watching a blu-ray with DTS sound may have had something to do with that. :)
Less impressive is the quality of the subtitles which often felt wrong (and has been commented on substantially on the spoiler heavy TV Tropes page).
As I said at the start Xam'd: Lost Memories is an impressive series. It doesn't feel excessively padded, is fun to watch (and listen to), and rarely triggers the tropes that I hate. It isn't the greatest series ever, but relatively few are better.
 For want of a better description.
 Popularising the genre being the only worthwhile result to come out of Evangelion.
 Although to a much lesser extent. It is mostly seen in the made up fonts, and the look/feel of the hidden village.
 Around episode 11 for the record.
 I can't help but think that this was a jab at the island sequence in Nadia: The Secret of Blue Water.
 Nicknamed "Cloud Rider" by the other characters to really drive this message home.
 The gender balance is pretty good throughout actually, as are most of the gender roles.
 And Nakiami had a long way to go in this respect.
 Whether, and when, Akiyuki realises this is an interesting question. :)