Finished Infidel this evening, after consulting the hints about just a couple of points (which turned out to be guess-the-verb-y things where we had the right idea).
Overall, I’m surprised by how relatively easy it is — the hieroglyphics puzzles are fun and consistent but not that hugely brain-teasing. It’s really easy to lock yourself out of victory by doing the wrong things in the wrong sequence, but mostly that’s about execution (remember to pick up your knapsack again before leaving an area!!) rather than about figuring anything out in particular. I found myself thinking that the emphasis on performance actually makes it a little more like a platformer than modern IF tends to be. It’s very hard to get to the end without having to replay parts — probably most of the game at least once, and some pieces perhaps multiple times — and even when you’re replaying it’s easy to screw something up if you drop the wrong thing in the wrong place and forget to pick it up again, or take a wrong direction by accident.
After a while it becomes a kind of proficiency run, to do all the necessary steps with no extras and no mistakes.
Emily Short's Interactive Storytelling 
(Spoilers here!)
I’m surprised you haven’t discussed what I think is, at least in an historical context, the most interesting part of Infidel: the ending, specifically the fate of the protagonist. To ask the player to play such an unlikable character, and then to kill him off even if you “win,” was brave and groundbreaking stuff in 1983, and a major step forward for IF as a vehicle for real storytelling. Even at this date, Infocom was somewhat ahead of many of its players. Many were decidedly uninterested in mixing tragedy or morality plays up with their text adventuring. An interesting transcript from a 1984 online Compuserve chat can be found at http://ifarchive.flavorplex.com/if-archive/infocom/info/berlynco_adv.txt, if you haven’t seen it already. Search for “did not like Infidel” for the good bits.
I considered talking about it, but I realized that (historically significant though it is and all) I personally had very little to say about it. The end had been thoroughly and completely spoiled for me well in advance — I mean, at some point I had read the entire end text, and the story of that ending had been told repeatedly before. So it wasn’t a surprise, and also didn’t seem to need more discussion simply because it existed.
You don’t have the problem with leaving the knapsack behind if you PUT KNAPSACK IN AIR while wearing it. Then it’s accessible to you throughout the game.
*Awesome.*
How did you acquire it?
This will sound suspicious, but I’m not actually sure. My family owned pretty much all of the Infocom games sooner or later by commercial means in the 80s and 90s, and at one point we put a bunch of them on my computer (several generations before the current one) and I’ve been gradually working through the ones I never played.
At this point I am not even sure whether that means we tweaked the files off the discs or whether we re-downloaded them from somewhere on the grounds that we owned proper licenses but wanted them in a better format; but there it is.
So if you mean “where can I get a copy?” the answer is “I don’t know”, and if you mean “did you pirate it”, the answer is “no”.
The first one. I lost nearly all of mine many years ago when we threw out the shoeboxes of 5.25″ disks for our Apple ][e.
The IBM edition ones took a little longer to disappear; occasionally I find a feelie buried in old junk and its like finding a picture of a relative who died young.
I fell horrible victim to the recent April Fools’ Prank about the re-release of Infocom games; I wanted it to be true so badly, it blinded me of common sense.