I've just played the whole thing through, enjoyed it, and would thoroughly recommend it, particularly at the price.
A Non-spoilery summary:
It's written using the Unreal Development Kit (UDK) and installs by default into C:\UDK\The Five Cores, taking up 624Mb after installation. It puts an item into your start menu at the top level to run it.
It installed with no problems on my old-ish Windows 7 64 pro laptop (Intel Core 2 Duo P8600 2.4 Ghz 4GB RAM, AMD Mobility Radeon HD 4570). However it can't cope with running with the display in full 1920x1080 resolution. It's fine in 1280x720 with all of the other graphics options turned up full. It still looks very nice at that resolution, obviously it has the slight chunkiness of realtime 3D, but it's quite Riven or Exile looking.
You can move around freely but it won't let you fall off the edge of the world and there are some places where you can get stuck, sometimes run-jump type moves will get you out of a hole, but once I had to exit and go back to a saved game. The controls, as mentioned in the posts above, are arrow keys or WSAD for movement, left shift for run (generally into something at speed) and space bar for jump. The mouse is for looking around and the left mouse button operates things. The pointer hand symbol only appears when there's something that can be played with.
You can save at any point, but there are only 3 save slots.
Like Myst, you operate from a main island and visit other places from there. Generally there's a puzzle to be solved to get there, and another to get back, when you collect one of the 5 cores. There are clues for all of the puzzles (with one possible exception involving combinations of 3 buttons) which vary from the blatant, to the slightly obscure ([spoiler]6:15[/spoiler]), it's not particularly difficult, but will keep you entertained for some time. Where a puzzle involves you moving between two places several times a convenient teleporting glowy thing lets you hop between them without trekking the long way around. There are a couple of vehicles on rails, elevators, switches to press, electricity, keyboards, colours, flames, things to rotate, dangly things to pull. What more could you want?
And it looks lovely.
If anyone needs a hint let me know.
Edit: Changed mouse button from right (wrong) to left (right).
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