Keflings puts the 'casual' in 'casual gaming'. It's an easy, very relaxed, but occasionally dull town building game.
Not much needs to be said about the gameplay -- it's a very straightforward town-building game: you collect and manage raw resources, building structures to better your people's town, collecting their love (which the town needs to grow), educating them, helping them trade with other (invisible) civilizations, etc. It's the epitome of casual gaming -- there's no combat and no time tables to do anything. This took some retraining on my part: I'm used to real-time strategy games where if you don't move fast enough there's some negative impact: you get attacked, your people get angry or run out of food, etc. But here, there are absolutely no negatives to taking as long as you want for any step of the game.
This lack of stress is a mixed-bag: No sense of urgency makes for a very relaxing game experience. You can wander your giant around, looking for semi-hidden tools to advance your building capabilities, help your Keflings do menial work, or just admire the progress of your game. You can build structures in just about any order (though there is a staged way to build sets of buildings -- you have to build some types to open up more types). Again, relaxing. However, that lack of any consequences to anything you do can lead you to a lack of purpose. I found myself pretty bored with the game after building my first castle (about 3 hours into the game).
However, my kids, aged 9 and 12, it's a good time. They love just meandering around, playing with their Keflings, building structures in certain patterns, etc. They can use it as a true virtual sandbox with no stress, and just have fun with it.
If you're looking for the next Simcity or Civilization, you will be disappointed. Keflings is definitely for the person looking to just relax and have a simple, fun building exercise, and in that, it succeeds.