Wow, the ending sequence is pretty impressive.
Overall, I enjoyed the mod. Not as much as the original portal, but the difficulty in some of the latest levels makes the original portal look like childs play solution wise.
My biggest problem with it was test chamber 19 - the last of the test chambers. The ending of that level requires you to climb a chimney using the portal jumping technique first seen in portal. There it was quite fun - a leisurely bouncing up and down from platform to platform through portals. In prelude however the chimney tunnel thing is so high you could fit the Combine citadel in it as a greeble. You look up from the bottom and you see nothing but platforms you're supposed to do the relatively easy bouncing technique between.
It sounds a bit long but not terribly difficult right? Well there are two other aspects to it that make it stupidly impossible. The first is that many of the platforms are moving - they periodically shift from position to position, which would of course remove your portal that might be on it. Highly annoying, but still not insurmountable really.
The killing blow is that you have to lug the companion cube up with you the entire way.
Upon seeing this I just went 'Uh, no.' and noclipped the entire way up. First time I've ever had to cheat in a portal map, which was very annoying.
The other issue is that some of the puzzles rely too heavily on your acrobatic technique midair, as opposed to figuring out the actual puzzle you're presented with.
Overall though - as long as you cheat through the test chamber 19, it certainly is all doable and I reckon worth the play.