This is my first time reviewing a game (Technically a mod) which is testament to how much I appreciated this mod.
It's a must play for any portal veterans, creating the closest experience of a sequel/DLC that we'll receive for a long time.

The added element of time (without giving too much away) is ingenious, and the restrictions with this new feature is cleverly thought out. Expect to be stumped at times, without a doubt some uniquely challenging puzzles are contained here which are sometimes cheekishly simple retrospectively. But, the mod does a fantastic job at stepping you through the concepts required to be successful at solving the more complex puzzles.

My only disappointment is that the gels don't make an appearance as a problem solving mechanic. Combined with everything here, it may have created an experience that could have felt even closer to the franchise.
Having said that, it's clear that this mod is an achievement.