I played the shit out of Portal; I bought it as part of the Orange Box on the 360 and, because my wife was in the TV room with friends when it came out, I ended up downloading it through Steam as well. I'm not necessarily a genius with it; I've gotten through a few of the challenge maps, but I my main pleasure in playing Portal is simply playing through the single-player campaign and absorbing the storytelling, atmosphere, and humor.
Portal's single-player campaign could arguably be the best single-player campaign ever, and one of the reasons why is because it's incredibly well designed. Its focus is razor-sharp; it does a fantastic job of teaching you how to play, so much so that you're not necessarily aware of it; the puzzles are challenging but never unfair; and the payoff - the final level - is absolutely brilliant. Up until the final level, you've been in very clinical environments, being hand-held throughout the game, and then - suddenly - you are seeing things you've never seen before and you now have to improvise your way out of your situation, because the stakes have been raised.
The commentary tracks in the game do a great job of explaining how all this came to be, and I recall being somewhat shocked about how much playtesting Valve does; they do playtesting pretty much on every single day in development, to make sure that everything makes sense and - most importantly - that everything is fun. I was shocked because I generally hate that sort of approach. You can spot it immediately in other art forms - movies, in particular, suffer rather horribly when they are influenced by too many cooks who don't know what they're doing. I felt that the amount of playtesting Valve was doing was somehow wrong, that at some point you need to have the original vision of the game come through without being diluted by so many opinions on such a moment-to-moment basis. That Portal turned out to be one of the best games I've ever played specifically because of this incessant testing seemed irrelevant; I felt like the creative vision of the game was being diluted.
I bring this up because I had been eagerly antipating the recently released Portal: Prelude single player mod, and the first thing that became clear to me - indeed, in the very first level - is that the reason why playtesting is so important is because it works, and I am unsure if anyone tested Prelude besides the actual people who built the game, if at all. Prelude is everything that Portal is not - it's not funny, it's unfair, and it's not fun.
Case in point. In order to solve Level 2 (!), you need to crouch as you fall through a portal, which grants you better momentum. Keeping in mind that crouching is something you NEVER do in Portal - indeed, I was unaware that there was even a crouch button in the original Portal - I would never have gotten past the level had I not looked online for a walkthrough. The ONLY way to solve Level 2 is by crouching, which is information that I didn't know I needed.
And what makes this incredibly unfair is that Prelude doesn't even tell you that crouching is beneficial, or that it even exists, until Level 3. That I had to cheat in order to figure this out, this early in the game, was incredibly disappointing. I'm currently stuck in Level 4 and even after watching a video of the puzzle getting solved, I don't really give a shit about finishing it - the solution is totally counter-intuitive and requires a degree of specificity that could only have come about from someone who already knew the answer.
I don't hate challenging games; I hate games that are challenging only because they're unfair. If the developers of Prelude had simply gotten 5 volunteers to play the game, at least 3 of them would have gotten stuck where I got stuck in Level 2 - indeed, they probably would've gotten stuck in Level 1, which I'm still not sure how I survived. They would see that the player didn't understand what to do, and then - if they were smart - they would have tweaked the puzzle to make it a little more intuitive, without necessarily making it less difficult.
Prelude is a free download, so the only thing you're losing is hard drive space. But you may also experience the crushing sadness of false hopes being destroyed.