Sam and I recently played the Alvin and Dexter expansion pack for the Ticket to Ride board games. My initial impression is that this is a fairly innocuous expansion that can be added to your game without a major impact. This expansion is usable with any Ticket to Ride game which is nice.
The basic gameplay is this: In addition to your normal turn you can move two monsters around the board. They can block you from building into a city and can affect scoring at the end of the game.
First, the actual design of the monster pieces is a bit suspect. They are a little more than topple-prone. They have tiny bases (by necessity since they need to fit into a city) and both monster figures are oddly leaned forward.
The affects on game play in a two player game were minimal. Really Sam and I only moved the monsters three times in our game. I believe there would be much more action with Alvin and Dexter in a four or five player game.
Players may want to consider strategy changes using Alvin and Dexter. Suddenly locomotives (wilds) become much more valuable since that's the only way to move the monsters. Many shorter routes may be more desirable than one large route to avoid having a big point route halved. Also, it may be a good idea to build a little beyond your destination to avoid making your destination city an obvious target.
Some alternate rules for this expansion:
Instead of simply halving a ticket when the monster ends in one of the destination cities on that ticket, why not cut all tickets in half that must pass through that city? This would promote players making alternate routes to avoid this outcome.
Perhaps you could assign specific powers to each of the monsters... perhaps Alvin can move further but Dexter can remove trains from the board.
Instead of locomotives, have the cost of moving the monsters scale like a train station in TTR: Europe - the first time is one card, the second two of the same type, the third, three, and so on.
Overall, this expansion is pretty "vanilla," - especially when you consider the concept of adding monsters to a train game. The rules are surprisingly sparse but the concept offers a lot of potential for home brewing some rules. At the end of the day, this expansion costs a mere $13.00 so how can a Ticket to Ride fan say no?