I've played quite a lot with colonies (even more than the base game), so here are their pros and cons in my opinion:
Pros:
1)You can pull off incredibly powerful turns, and usually every player gets such turn. Compared to colonies, a -5 discount on everything is not that scary
2)The order of playing cards starts to matter a lot and rewards precision and knowledge of the deck, though the game slows down a bit.
3)There's always something to play. Almost all your money gets used each turn.
4)The microbe + animal combo becomes much stronger (especially with the promo card Meat Grinder: blue card, costs 5 MC, has a building tag; provides 2 MC every time you gain an animal resource).
5)Very high scores are possible. Our record is 175–354.
6)Earth becames extremely strong
7) 100+ MC or 10+ TI prodaction is not uncommon
Pros or cons depending on the player (I like #1 #2 and OK to #3 #4):
1)Terraforming becomes unviable; engine building gets a major boost. In particular, heat production is useless, and plant production is only as part of a bio combo or with crazy discounts.
2)Games last longer. My stats for 2-player live games: 3.5–4.5 hours with colonies + Venus, 2–2.5 hours for a standard game.
3)Manutech is crazy OP
4)As a result of point #1 and the addition of very powerful cards, games become less competitive.
Cons:
1)Because terraforming is not effective, games can become very one-sided, and there's no way to end them quickly. After 25+ games, we even started conceding to finish early.
2)Playing with 4+ players is impossible; 3 players is also not great.
3)Due to the large number of blue cards, there are situations where one player has passed, but the other still plays for another 15 minutes. In a game where my opponent scored 374, I passed and then helped him finish his last turn — it still took over 30 minutes, and the whole game lasted about 7.5 hours.
4)Early energy production can be too good
