I would say its a puzzle game with city builder visual theme, and easy to understand game principles. Basically you are placing buildings with adjacency bonuses, on various small islands, you get score for it, unlock more buildings to place, and so on.
It is comfortable to play, while maintaining satisfactory level of increasing difficulty, and that is probably the reason, why you can spend hours playing it. And do so without being bored, and still feel like relaxing.
Visually its well done blend of simple models and textures with smooth lightning and rendering. Designs of puzzle pieces (buildings) are easy to recognize, and feel warm and sharp. Its mostly static, with a few, mostly ambient and interface movements.
Music and sounds are good fit, and support the general feeling of a game, but that is all.

For what can be improved - placement of buildings could be more spacially flexible, especially when you lack some very thin space to place something, that looks like should fit alright.
Also building variations will wear off after some time. And authors could implement some feedback from players in terms of bonuses and adjacency range - it could be improved to generate even more believable city structure.

For current low price Islanders is very nicely done game, and so I can recommend it.


Also I want to point out its download is just 120 MB large! Which is absolutely unseen in these times, even for a games with less advanced graphics and content. I know it has no impact on gameplay, but still impressive feat.