The first game that I have heard of that uses Terry Pratchett's Discworld as it's theme is Guards! Guards! Because of the theme of the game, one of my friends immediately decided to buy it at GenCon, and I was honored to get to play it with her.
In Guards! Guards! the library has lost several of it's spells, and they have been scattered throughout the city. Since this job is far too big for any one individual, each of you has been assigned only five of them to return - indeed, even your share of spells is more than you can handle, so you go around recruiting allies. These allies will help return the spells to you once you have found them (navigated to them on the board). You must be careful, however, as some of your opponents want to look good by returning their spells first. Therefore, as you send your allies to return the spell, they might encounter some of your opponent's saboteurs who might attack them, or even try to convince them that they are working for the wrong person. After your allies manage to return to the library, the spells still aren't immediately returned - the library has defenses to prevent anyone from just walking in! You must pass various skill checks (depending on which spell is being returned) in order to get it all the way into the library. The first person to return all five of his assigned spells is the winner! (Oh, and there are also dragons and crazy luggage in the game.)
The first thing that I must confess in this review before writing any pros or cons is that I am not really familiar with Discworld. I have never read any of the books, and so I am forced to review this game only on it's merits as a game. I can tell you second-hand that the references to the books are quite amusing, and that my friend who owns the game thoroughly enjoyed reading through the ally cards, looking at the art, and reading the blurbs about each person.
The first pro that I see to Guards! Guards! is the crazy, runaway luggage. Specifically, the luggage is looking for it's owner (it won't find it), and so it is wandering around the city. And, unfortunately for you, it is apparently much bigger than you, as if you are in the same place you suffer enough injuries to go to the hospital! This is an interesting mechanic that is wrapped in a goofy theme and seems to work well. Each time a new ally joins someone's party, the luggage is moved a certain number of steps around the board (and his path has several forks in it that force the current player to choose a direction). Whereas this sounds insignificant at first, it is amazing how much a player's movement will be influenced by where they land on the path relative to the luggage. And, moreover, the person that doesn't pay attention to this may find himself in the hospital more often than not.
My second pro is that the game has dragons. I really like dragons and feel like they should be in more games. However, with Guards! Guards! you don't actually encounter them very often (in my limited experience). But, when you do encounter them, they can drastically alter the game! A quarter of the board will become fairly useless until the players choose to band together to defeat it (which is no easy task).
Now for the first con - I absolutely hate the end of game mechanic. On the final spell, you are forced to make a Magic 9 and a Guild 9 skill check to return it. This means that you roll an eight-sided die and add your Magic value (an attribute that you can increase throughout the game but that has a maximum of +5) to it - and this number must equal or exceed 9. Next, you do the same with your Guild value. Magic and Guild are very challenging attributes to upgrade. Some of the allies give you boosts to these stats, and when you return a spell you can choose to upgrade one of them as well. However, even if you get all of the possible upgrades, it still boils down to a die roll. And, since the games I have played have actually been fairly even between players, this ends the game with the players simply taking turns rolling until one of them eventually gets the die rolls that they need to win. Yes, the person that has more upgrades has a better chance of winning, but poor die rolling can indefinitely keep him from that goal. The more I think about it, the more this end of game mechanic reminds me of the one in Killer Bunnies, which annoys me so badly that I won't play it (Killer Bunnies)! At least in Guards! Guards! you are able to feel like you are in charge of whether or not you win, since you're the one rolling the die.
The next con to Guards! Guards! was simply that the game wasn't especially engaging. More than anything, it seems to be a race around the board to see who can hire the better allies and return their spells the fastest. Yes, there are some ways (like setting up Saboteurs and running them over with Luggage) to affect your opponents, but strategically, it doesn't always make sense to slow them down at the cost of returning your own spells (after all, a Saboteur can't return a spell for you while they have an ambush set up for your opponent). Some things affect your race to return the spells, but I think that good die rolling (when charming allies and returning spells - you can move 6 spaces each turn and don't roll to determine that) and which allies you have the opportunity to hire affect who wins the game much more than anything else. I simply found myself disengaged with Guards! Guards! each time that I played it.
Overall, I give Guards! Guards! a 6.0/10. Obviously, if I enjoyed the Discworld lore, this number would probably be higher. However, from playing it strictly as a game I was very disappointed. There were a few neat aspect to the game, but in the future, I would just assume play something else.