You can earn items and money to transfer into Fable II, which is a rather clever idea.
There are lots. The first, and most obvious, is that without using the (in)famous glitching system on the Fortune's Tower game mode, it's next to impossible to earn money - at least, impossible to earn it in amounts that actually mean something. What's the point of having a game where the entire marketing line was "transfer your winnings into Fable II," and then you don't actually get any? The games, like real casinos, are punishingly hard if you are trying to play them legitimately, and without the glitch it is very easy to end up in significant debt.
While the idea of transfering some items into Fable II is a nice one, what isn't so nice is the fact that a few of these items are obtainable only in Pub Games, and you cannot get Fable II's "complete the game 100%" achievement without them. So, in essence, if you want that achievement, you need to buy both games. Microsoft needs to address this.
The other big problem with the game concerns a couple of the achievements. Two of these achievements are like the "Numeric God" achievement on Soul Calibur IV, which is to say they are completely random. They are the type of achievements that you could hypothetically get in 10 minutes; or you could play for 100 years and never get them. There is no skill required, which is infuriating.
Fable II Pub Games is easily a game to avoid, unless you don't have to pay for it. The exception to this would be if you're willing to use the glitching method to make money for Fable II; in that case, it might be worth it. Otherwise, give it a pass.