If I had to define lain in a single word, it would be captivating.

This anime easily has one of the best artistic directions I have ever seen, along with Evangelion, Tenshi no Tamago and Akira.. Beautiful colors and designs, and a dark and depressing setting that I love.

I feel like the only thing I didn't really like about lain was its very limited variety of interesting characters; I mean, Lain and Alice are great characters and are extremely well written, but what about the others? I felt like the others were just there to get in the way, really, since the entire plot is centered around Lain and revolves around her.
Lain's evolution throughout the series reminded me a lot of movies like Thanatomorphose or Tetsuo, which I didn't expect and ended up falling in love with. Lain ends up practically becoming software, and ends up being practically eliminated from the real world. It is quite complex to define, but it is undoubtedly a special and unique masterpiece.
It's quite surprising how the anime hit on a lot of things that ended up happening in the real world decades later, such as dependence on technology and isolation. Things like that make me realize how brilliant the story and its script are.

What an incredible experience.

The anime touches on very recurring themes in Evangelion, which of course I loved. Lain is extremely similar to Rei Ayanami, for example, both in personality and history. Lain and Rei are girls who were manipulated in one way or another and who ended up becoming numb. While Rei Ayanami serves as a destroyer of the bridge —known as Instrumentality— that connects the body and soul of humans, Lain would be actually the bridge —known as Nexus— that connects the real world and the "virtual world." They are polar opposites but at the same time they are extremely similar. Also, I loved the topics of the disconnection between body and soul, disappointment, abandonment, the useful life of a human being, existence and philosophy. Without a doubt, lain touches on quite complex themes that captured me from the first second.
lain really made me think too much, especially since episode twelve, when Lain betrays Masami. Much like when Rei betrays Gendo, Lain betrays Masami because she realizes that emotional connections are important and necessary. Having a physical body is necessary, even if this makes us weak or imperfect beings. Masami called this betrayal a "virus in the software", since he planned for Lain to destroy the Nexus to unify the real world with the Wired, however, Lain, like Rei or Shinji himself, understood through love that abandoning the physical body is not evolution but rather a step backwards. After this, Masami formed a body to try to stop Lain's decision, what he called a "virus", creating a physical body to stop her, completely contradicting his ideals that having a body is useless.

Without a doubt, I highly recommend watching and understanding Evangelion's logic to understand lain's, since both stories start from the same premise that human beings are imperfect and need to evolve by abandoning their own humanity, feelings, and emotions.