I have the last 3, We Are the Others, Interlude, and The Human Contradiction. Obviously, Delain's very similar to Within Temptation, with the two bands' masterminds being brothers. Delain's music if anything might be a little "safer" than WT's, in the sense that they never go completely off the rails (Xzibit, anyone?). I also like the fact that Charlotte doesn't attempt to be a Tarja clone like so many of the female singers in the genre, probably due to the fact that she grew up singing jazz and pop. She also comes off very likable and down to earth in interviews, and the fortunate people on this thread who have met her say the same things.
WATO and THC are both solid albums IMHO, with only a few duds sprinkled among the two of them. Interlude is an interesting compilation album. There are a couple of re-worked songs, a much slower WATO which is worse than the sing-song original, and a just barely different Are You Done with Me (stronger backup vocals, I think.) There are some pretty strong live tracks, although if you listen to some of the songs individually they are cut up rather oddly, with music from the previous song still playing at the start of a new one, etc. There are also some outside the box cover songs, from bands like Talk Talk and Dresden Dolls, and a Cranberries b-side. Interlude also comes with a bonus DVD (regional code 0, unfortunately, so I have to play it on my computer) that includes some of their old videos and behind the scenes footage.
I haven't bought L or AR yet, mainly because I haven't seen either of them in stores since I've become a fan of the band, and L is probably obscure enough that I probably never will, unless the band has a breakout like WT or Nightwish and their older albums get re-released. The two albums are available on iTunes, where I've downloaded a few of their songs (the ones that still make their setlists, like Pristine, Sleepwalker's Dream, AR, and Go Away). L has two versions of a song called No Compliance, the main one being a duet between Sharon den Adel and Marco Hietala, while the bonus version has Charlotte singing instead of the other two. Even though I normally like Sharon slightly more than Charlotte, I like the latter version better because Sharon reverted to her Enter/The Dance/Mother Earth banshee mode for this song, which is sometimes pretty jarring to listen to, while Charlotte sang it smoothly like she does anything else.