I've recently been playing an iOS game called Sonic Dash, and it is a good example of how to do freemium right. While not the most original game (it is--at its core--a Temple Run clone), it is a fun game that distills the core experience of Sonic the Hedgehog games--running, jumping, and loop-the-loops.
That speaks to the fun, but what about the freemium? Well, that can be summed up by saying that Sonic Dash, while clearly trying to sell in-app purchases to the player at various points throughout the game, makes its sales attempts in a way that is much classier and more enjoyable than many other freemium games.
Take Words with Friends as an example. It not only has a user interface filled with distractions (mostly ads), but degrades the user's enjoyment by taking over the entire screen with ugly ads. In the worst cases, Zynga takes the cake by forcing users to load a video ad.
In contrast, Sonic Dash integrates in-app purchases in a more sensible manner. When a user loses, they see this screen:
The user is presented with a sales proposition (in this case, to purchase tokens that let them continue their current 'run'). Notice the difference from Words with Friends. The ad is something that helps the user continue in the game, rather than being an ad for something completely unrelated to gameplay. As well, the ad can be quickly dismissed by tapping the green arrow.
Tapping that green arrow, by the way, presents the user with the following screens:
The user is presented with items that can be purchased using 'currency' that can be earned via gameplay (in this case, collecting golden rings). As well, the user is shown their score for the current 'run'. Tapping the 'X' on the hint window will take users to the final screen, where they can share their experience on social networks, see a list of items that can be purchased, or simply play again.
It's important to take note of the fact that not only is the for-pay portion of Sonic Dash more sensible in the context of the game, its ads are also much more pleasing to the eye than the typically garish ads displayed in Words with Friends.
Folks, SEGA has done a fine job of producing a freemium game that is fun to play and sells items without being distasteful about it.