I've been playing Terraforming Mars a lot, and it's a game that has stood the test of repeated plays. I like the balance between having to stick to a strategy and having to remain flexible to take advantage of the cards you have available each generation.
The game can suffer a bit from a slow start as player get their economies set up. The first few generations of the game can be a bit dull, as no-one has enough cash or other resource to get the more powerful (and therefore more expensive) cards to the table. By the same token, production of plants and heat is so low that the oxygen and temperature global parameters hardly move. It's typically after about four or five generations that the game starts to come alive.
The Prelude expansion does a lot to alleviate that. The prelude cards (with fetching magenta trim) provide a kick-start to your economy. Most of the cards give you some kind of production; some give you a bunch of resources as well. It seems that this is enough to eliminate a turn or two at the start of the game.
At the start of the game, you're dealt four of these Prelude cards and have to pick two of them. After everone's declared their corporation and bought their starting hands, players play their Prelude cards in turn before starting the first generation proper.
These cards introduce even more choice at the very start of the game. You still have to choose one of two Corporations, buy three or four of the ten initial Project cards, and then have to select two of the four Prelude cards. You're making all these selections at the same time. This is great if you know what all these cards do and how they might complement each other. But I worry about how new players would react to this.
The beginner corporation is great for new players because, at the start of the game, they don't have to make any decisions about which cards to keep. This is important because new players don't have any real idea what any of the cards do. I fear that giving the Prelude cards to first-time players will overwhelm them even more than they can be already.
But if you've already played the game once or twice, Prelude is a great expansion. The Prelude cards give you the ability to fix on an early strategy and get a good way into makeing it work, right at the start of the game. It makes the game a bit shorter, but only by eliminating a couple of boring turns at the beginning.
Prelude is a great expansion for Terraforming Mars, right up there with the Hellas and Elysium alternative maps. If you like Terraforming Mars, this expansion just makes it better.