After managing to work through a few bugs (detailed in a reply to the changelog) I finally got through to the end.

I enjoyed the early game. Finding the IP to pass the trials was fun - a little easy, but suitable for the start of the extension. I wished there was more out of the box thinking later on to match.

I didn't much care for Polybius though. Early attacks like defacing various news sources seemed childish for a person in their position. The reasons why they're lashing out like this become clear at the end of the game but it feels like managing to get into this great team only to then perform hacks more suited to the initial "l33t" IRC group. Later actions only paint the character as an angry kid lashing out at anyone that doesn't give him what he wants - which is probably intentional, but I felt less and less like working for this character. This lead me to unintentionally break the extension when I took an action of my own initiative rather than follow instructions. (detailed in the changelog comment I made)

The player doesn't get a valid JMail login, leaving the only way to check mail being the icon in the top right - when it's not pointing to an IRC room. there was a point early on where I received mail, but missed the chance to read it as I finished a mission and the icon switched to an IRC connection.
Added to that, there are several JMail credentials you can find during the early game, but none of them are valid accounts. Perfect fodder to reward the more curious!

Computer security structure really needs some variation. Every hack was breach as many ports as you have tools available at the time, with the only variation being whether a server had a proxy or not. The proxy was a non-issue, as you are given ComShell and OpShell relatively early on - making loading up as many shells to overload the proxy in no time trivial. It becomes quite egregious near the end as there are 5 systems to breach in a row within 11 minutes - each with 8 ports to open and firewalls, and running out of time apparently leads to having to redo all 5 systems. I passed this on my first attempt, but I was quite sick of running the same port cracks in the same order with the only variation being the firewall password. This mission also serves as the only usage of the whitelisting daemon feature - not an issue to experienced players, but this makes for a very poor time to reintroduce the system. Having an earlier mission familiarize the player with the whitelist daemon would help prevent a player from feeling cheated by the mechanic being thrown at them under a strict time limit, with a high price of failure.

I'm personally not a big fan of being unable to prevent incoming hacks. The shell has a trap feature for those of us that are prepared to prevent these! Polybius has a plot reason for this, but I found his 'tether.sys' early on and removing it did not prevent these, only broke a scripted sequence for me later - detailed in the changelog comment. It prevents me from being able to 'outsmart' these hackers, instead I'm left feeling I'm just along for the ride, playing a visual novel.

I liked the idea of one such moment renaming all your tools to l33tspeek in concept, however it didn't work for me as I rename all my tools already to speed up port hacking. The only real solution to this would be wiping out the players' /bin folder, but even in the worst case I would have just used one of the many filedrop servers to reclaim my tools.

I accidentally got the Polybius Spared ending as I went to reopen a proxy after connecting. I feel some other check should be in place there, such as replying no to the mission email checks whether you breached security of Polybius' machine first and chooses the relevant ending.
Ultimately there's also no way to expose Atlas to the authorities, or NH making the ending feel hollow if you take any other path than refusing the mission to destroy Polybius - and even then, that choice would leave you with the least answers.

I hope you find what I had to say helpful!