SimLife is a full-on simulation game, and very unlike a lot of Maxis' others.
It's not exactly a game in the way that SimCity is, but more like Flight Simulator perhaps. Except completely different.
As someone with a keen interest in ecology and genetics, I find it fascinating, but treat it more as an educational tool. It really helps you think about large scale ecosystems, and the interactions between physics and biology.
I've sunk quite a few hours into it, but fully realise for most people its a complete turn-off.