A good balance of challange within naturally and organically designed levels. It reeks of a classic half life 2 feeling, and takes after the best of it's combat. Parts of the maps still resonate in my memory, even months after playing it.
A free mod is The Citizen. A great half life two mod. One of the best! But, that is how much you should get for a free mod. Prospekt comes with large and original maps, a couple of hours of gameplay, and hundreds of custom assets. It's worth money. It's it's own game.
In Gaben's own words, "What's not to like?"

Beautiful.