Plays: 3P (2 humans 1 robot) x1.
I've always liked Power Grid, and own most of the expansions. Unfortunately most of the time it's just Michelle and I playing 2-player games. So I was immediately interested when I heard about the Robots expansion which introduces robot players. It allows players to add robot players when the number of humans is low, to make the game more interesting.
Each robot is made up of five parts, and each part specifies some rules governing the behaviour of the robot. The parts can be mixed and matched, resulting in many possible combinations and robot characteristics. There are also some base characteristics of the robots, which they will follow precisely unless overridden by their special characteristics. During gameplay, players need to manage the robots, and in some cases make decisions for them. The robots' actions will not always be optimal. They are there more to be exploited by the humans than to compete. Their actions are mostly predictable, so a clever player will try to use it to his advantage.
Adding a robot does feel like having an additional player, albeit not a very threatening one. 2-player Power Grid is not as good as 3+ players, so in my case it is nice to have some new dynamics. Phase 2 no longer means all cities are opened up, because the robot can now be used to occupy the second city spot to block your opponent until the start of Phase 3. Taking into account the robot's behaviour is important, and manipulating turn order is important. The cost of having an extra player (or more) is the overhead in calculating the robot's actions (most of which are deterministic) and in managing the actions themselves. Sometimes the robot can be a useful tool to hinder your opponent.
Power Grid is still Power Grid though. The game doesn't feel very different. This expansion is not meant to significantly change the gameplay. In the game that I played with Michelle (and one robot), I actually came last, i.e. I lost to the robot! But I swear I was not beaten by it. I was just outmanoeuvred by Michelle. She had quickly connected 17 cities to end the game when I had not expected her to be able to afford it yet. I keep losing to her at Power Grid. Must be the accountant in her.
I think Power Grid: Robots achieved what it set out to do - to allow robot players to be added when there are too few humans for the game to be interesting. If there are enough humans, say four, or even three, I'd say the robots are not necessary. There is some overhead in managing the robots, but it's worthwhile if it lets me play some interesting two-human-players games.