After playing 4 times (2 wins, 2 losses, one of us had a science win, the other had a military win), I took the time to write up some more 7 Wonders Duel thoughts. I posted these to my boardgamegeek review thread as well.
General
Overall, I feel 7 Wonders Duel is a bit like Splendor on steroids. For the unfamiliar, Splendor is a 2+ player game, compared to Duel's 2 players, that mostly involves buying cards and collecting tokens. Owned cards help discount further purchases. In Splendor, you can sometimes block your opponent(s) in some fashion, and you watch them in order to decide your own moves. In Splendor, sometimes luck is against you, and your actions may reveal cards your opponent(s) want.
Similarly, 7 Wonders Duel involves collecting cards and money, which in turn allow you to collect more cards, more money, and some other bonuses. You can block your opponent by taking cards they need, and you should pay attention to what they are doing, but sometimes you can still have bad luck and reveal something they want.
Should you buy the game?
I feel nearly all games depend on the players involved, and this is no exception. There are at least four aspects that could be either pros or cons for you. If you are okay with these, I recommend the game.
1. As the name suggests, this game is a duel. The original 7 Wonders was a bit like multiplayer solitaire with occasional neighbor interaction or card removal. I actually did not like the Cities expansion because of the gold-taking mechanic. Duel focuses the players against each other, and you can hurt your opponent in a variety of ways (starving them of resources, raising prices, taking cards before they can, making them lose money, etc.). This is good if you like attacking your opponent, and bad if you prefer solitary gameplay or become frustrated easily. Takeaway: Both players need to be okay with being attacked and losing.
2. As with the original 7 Wonders and Splendor, this game is not pure strategy. On average the more skillful player should win, and there is a mechanic to help the losing player, but sometimes you may simply have a bad deal. Some cards are hidden, and your opponent may turn over good ones. Each game is played with 13 cards (3 from each age, 4 guilds) randomly removed, and you will not know which ones until it is too late. One game, 2 out of 3 wood resources had been removed, making wood surprisingly scarce. This is good for variety, and bad if you prefer pure strategy. Takeaway: Both players need to be okay with some luck.
3. If you want to win, you cannot marry yourself to a strategy. No matter how cool progress tokens and science are, you will want to look at the board layout and carefully consider your options. One game, you may be able to starve your opponent out of resources early. Another game, you may both have lots of resources, and need to adjust your strategy to focus on wonders or military. Otherwise, if you always focus on military for example, you may end up getting destroyed by certain card/wonder/progress combinations. This is good for variety, and bad if you really like certain strategies. Takeaway: The best strategy can vary, and both players need to be okay with this.
4. There are several things to keep track of, and the game can be stressful and tight. Sometimes a single card mistake can lose you the game. If you ignore military, your opponent may win the game early, or at minimum cause you to lose money. If you ignore science, your opponent may win the game early, or at minimum gain valuable progress tokens. This is good if you like balancing game aspects, and bad if you prefer a simpler focus. Takeaway: Both players need to be okay with juggling different game aspects.
Overall, I like the game. I hope this helps you decide for yourself.