Just played through Portal Prelude again for the first time in 2 years. Despite it's obvious flaws, it's not a bad mod in the slightest. Here's the breakdown:

The Good:
The biggest thing this mod has going for it is the coherent story. Granted, it's way off-canon, but it's internally consistent and actually rather compelling, once you get over the terrible TTS. Aside from that, some of the puzzles were legitimately well made and had great, portal-theme difficulty. The mapping is pretty high quality work, too. And if you don't mind racing around with bunny-hops, the last map can be legitimately fun and terrifying.

The Bad:
Given that this is a portal mod, the expectation is that you've played Portal before. However, the difficulty curve is much too steep: it starts out with button and box testing and by chamber 6 you're racing a crusher and electro-beams. Speaking of which, the implementation of these is quite terrible, it's difficult to tell whether or not you will die when you enter it.

The Ugly:
The voice work is horrendous, needless to say. Even if they had done english with french accents it would have been better. The script is pretty poorly written, especially during the ending sequence with GLaDOS. "What are you doing with that? I don't think you're doing what you think you're doing. Anyway, do you even know what 'this' thing is?" What is that even supposed to mean? Also, the difficulty in the midrange chambers stems completely from stupid turret situations. It's worst near the beginning of 16, where there are two turrets just behind a fizzler, over a ledge. You have to portal up onto the ledge, go over the fizzler (at which point the two turrets have started shooting you), and get behind the turrets before death, which takes about 3.5 seconds. Situations like this become extremely frustrating and are no fun at all.

So in retrospect, despite it's flaws, it still comes off as a very high quality mod. It certainly made a legacy for itself and it got me into Portal mapping, as I'm sure it did many others. The soundtrack was great, very portal-ish yet properly emotional (go THA!) and the Chemical Brothers makes the best boss fight music.

I recall we had a project started a while ago to modernize this to Portal 2 with the updated plot details. Maybe we should consider taking that project back up at some point, who knows?

I like the concept of really hard puzzles, but it needs nothing less than a total overhaul for everything else. Also, a different name.