POSSIBLY CONTAINS SPOILERS:
Much like the Neverwinter Nights scenarios I downloaded, this far exceeds the original game (which, again, like NWN, is supposed to be a tutorial/example of using the game mechanics) in terms of difficulty and fun. Then again, the amazing game mechanics and ideas became a backseat to the wonderfully dry and black humor of the original. For me, at least.
Interestingly enough, some of the rooms I only beat because I've played the actual flash version first, and, thus, had a better idea of what I was stumbling around in, especially the replication of Room 40. The new levels I found fun, but they didn't seem nearly as challenging as the old TFV levels. Without previously playing the actual Flash version, the replication of Room 40 would have had me stumped for a week. It's much more difficult when you don't have the whole layout of the room in front of you.
What I think a lot of the review posts have not touched on, however, is the very clever use of GLaDOS' voice clips. I thoroughly enjoyed the "The Enrichment Center apologizes for this clearly broken test chamber" moment.
And the boss certainly elicited an "OH HOLY CRAP!" moment from me. Oh, and, seriously? A time-pressure fling puzzle over death-water? That's just mean.
Excellent work. I certainly hope you come out with more.