I did a 6 ish hour journey on one of those (Katowice-Gdansk - was going to Gdansk anyway so flew to Katowice the day before to have a go) and I thought it was an excellent unit, bodes very well for East Anglia's ones.
Unfortunately they are also exceptionally unreliable, although knowing PKP, I wouldn't neccessarily put that down to being the fault of the trains themselves. PKP is having to substitute a number of them every day, check out the announcements page on their website for replacements.
https://www.intercity.pl/pl/site/o-nas/dzial-prasowy/komunikaty/
If you go back a few pages you will see a lot of stuff being subbed for very old and much shorter trains, that cannot carry disabled passengers and they're also (as I found out) not honoring seat reservations on the older stock, cannot take disabled passengers as they're step entrance only, no catering, and can be the lowest of the low in quality of trains, no plug sockets, no wifi four seats squashed next to each other in compartments and over 30 years old.
The problem with PKP, as someone who has used them for ten years, is if you are lucky enough to get a modern piece of rolling stock that everything works as designed you will have an excellent journey normally for a good price. The problem is that doesn't happen nowhere near often enough I've been on 10 PKP trains this week as part of work, and they've got it right three times.
I've had two FLIRT substitutions, one train that shook so much I thought it was going to derail, another one with ac stuck on heat mode and nobody can turn it off, a train which every carriage had same number, another one which all the onboard systems was stuck at 10am on the previous day and kept saying Nastepna Stacja / Next Station Warszawa Centralna for 45 minutes every 60 seconds, one with no working lights, another with windows stuck open, the list is endless.
They have another unit called the PESA Dart as well introduced around the same time as a flirt. All I can say is this. How a modern train can feel claustrphboic I have no idea, but this train manages it, it's the worst modern built EMU I have seen, the interior is quite frankly ****. Carriage 4, the conductor one is even worse. there are so many walled off sections of that carriage, and divider walls and big thick obstacles between seats, that it feels totally horrible.
Interestingly 1st (which, at the pittance fares PKP charge, was where I sat) used the same or similar seats but in 2+1 with double 1st-sized centre armrests and recline (*not* the old-fashioned-style GWR 1st seats!). I wasn't sure about how this looked, but I actually found it very comfortable and it actually felt very spacious.
The other thing about first class on those FLIRTS is that the banks of two seats on the 2+1 have a full sized table for each seat and those tables are massive, much bigger than you find on 2+1 in the UK First Class.
Operationally it seemed a little curious, there was a guard, but "dispatch" apparently involved radioing the driver using a handheld radio to tell him when to close up and depart (using the typical EuroFLIRT wing mirror and interlock approach).
Operationally PKP is generally weird. Some of the stuff I've seen over the years is incredible and I would have very little confidence in them overall. They get stuff right more often than they used to, but still nowhere near as much as European standards. The number of sub-brands of their services is silly too and their timetables generally are all over the place with no real frequency basis.
As a company they have certainly improved but there is still weirdness there such as silly track/platform numbers (like 202/304) a truly shocking online ticket buying service, an English language website which is hopelessly out of date compared to the Polish - I'd go as far as saying a Google Translate version of the Polish is VASTLY better than the actual English.