I've been enjoying a lot of the Euros that are available for the iPad, here's a list of what I've tried and how I think they stack up:
Mu: Maybe my favorite. Mu is a great card game for five, but it has the downside of having a bit of a learning curve. You see, you bid to communicate, not only to get to declare trump, but also to let the high bidders know that you might make a good partner for the chief. The problem is that it takes a few hands to get used to the system, and it's hard to explain, harder yet to figure out in the first place if you can't practice.
Now you can. I've played two games to 500, each taking about an hour to complete, losing both (taking second and third). The first game I lost when I somehow ended up being "on the hook" after my bid, which I didn't realize I was doing. Don't expect the game to coddle you too much, although there is a "Shake-a-Hint" system. You can set the computer player skill level, points to win, and how many players are in the game. I don't think you can play it over a network, though.
I should mention that the game does *not* support the "Mehr" part of the boxed game, however.
Now if they could just make Tichu an iPad/iPhone app...
Money: Definitely down the scale of complexity, this is still a fun 20 minute "filler" game. Again, you can set your opponent difficulty and the number of players. While I like this app, I like playing with real people better. One rule I'd forgotten: You can swap out with one of the sets your opponents put on the table, not just the two pools on either side of the draw pile.
Small World: I don't have this game, as I preferred Vinci over the remake, even with the static map. I hear it's good, but there's no AI and so I'm unlikely to use it.
Keltis: This is the game that eventually became Lost Cities: the Boardgame. I think. You can play with up to four players in a hotseat configuration, which would be very useful for trips. Again, not something that I've tried out much yet, although now that I've got Mu down this will be the next thing I try.
Poison: Definitely a game that's better on an iPhone. A great time-filler, and this was my go-to game until I figured out the interface for...
Zooloretto: I love the board version of this game, and I love the iPhone version. It takes a while to learn the interface so that you can see what your opponents are doing, but once you do it's perhaps my favorite iPhone game.
Catan: I'd like this more if the computer players would trade with me. Even using a "dice deck" the numbers still seem to come out in their favor. I also got a random board that took forever to play because it was so hard to get wheat (3, 11, 9), and the one good wheat space got towns on either side early. This game needs a Conquer rule.
Ingenious: Also an iPhone app. I like this, but it only plays with two (although it can be AI or hotseat). Also, the Tournament settings don't work on my iPhone for some reason. You can adjust AI skill, or even give the game a time limit if you are feeling masochistic.
There are quite a few I haven't gotten around to playing yet, but (Small World aside) these are all on my iPad, and a couple on the iPhone as well.