My initial thoughts:
Hardware
I ordered a mini-HDMI to HDMI cable from PiHut because I don't have a mini-HDMI adapter in all my stuff. The cable is about 30-40cm log so it would probably make do most of the time, though I would have preferred a mini-HDMI to female HDMI socket, at which point one can plug in the HDMI cable they already have.
The Pi 500 is powered by USB-C and comes with a USB-C 27W power adapter with a moulded cable. I would have appreciated a socketed cable so that whatever length power cable can be more easily used. I realise that such power adapters are more commonplace these days, but these extra bits would have been more usable / re-usable if not for such limitations. The cable is about a metre long as well so it's not hugely generous.
The keyboard itself has been fine so far, feels like an average laptop keyboard.
Software
The OS boots up reasonably quickly, about 20 seconds I think. The initial setup insisted that my username should not have an initial capital letter (ooo-kay then!). No issues with user-friendliness there. It asked if I wanted to use Chromium or Firefox as my primary browser and also asked if I wanted to remove the other browser. It also did OS/software updates straight away which for newbie users is a good thing.
This Pi 500 came with 32GB storage which was 19% full at the point of getting to a usable desktop, which is a damn sight better than what one can expect for Windows. The user interface is spartan / simple, which makes sense given the limiting hardware. I immediately put the taskbar at the bottom of the screen (top is default) because that's my preference (easily done, right-click on taskbar, change option). The Start / app launcher menu is also very basic and simple.
Power options / sleep mode
Something that surprised me a bit was the lack of sleep mode as an option. You can shut down or restart. I also accidentally left the computer running for a bit and I was left with a black screen with something like 'plymouth-shutdown.service' so the only choice I felt I had was to hold down the power button until the computer switched off. IMO the lack of sleep mode is a serious oversight, and at some point I must leave this PC running for a while again just to see what happens. I haven't yet spotted options for switching the screen off after say 5 minutes either which would constitute a more serious oversight than the lack of sleep mode. Perhaps I could achieve both with a bit of Linux-Fu, but my initial perspective / plan for this computer was as the kind of device that could just be given to anyone and say "off you go!".
UI / browsing performance
UI responsiveness is generally fine. Firefox starts in about three seconds (I think Chromium starts quicker, though I would like to test that theory more thoroughly), and on one occasion when I closed Firefox and re-opened it, it evidently didn't have time to finish closing so it complained that it was already running. Web browsing performance I'd say is a match for some Athlon II X4 / Phenom II X4 AM3 Windows (or maybe a similar gen i3/i5) installs that I did for customers that have 8GB RAM and a decent SSD; I've definitely seen web pages load faster, but I think as long as performance is consistent then the only people it will bother are very impatient people. The fact that it runs Linux in my experience means that performance is likely to be very consistent. I would like to see if it would perform better with a decent NVMe drive instead of SD because I know for a fact that SD-booting Windows laptops perform noticeably worse than even the bottom-of-the-barrel NVMe drives that most big-name laptops come with. The Firefox install comes with UBlock Origin by default, kudos to whoever thought of that because I anticipated that being my first port of call!
Add / remove software and LibreOffice
I used the 'Add/Remove Software' feature in the app launcher menu to install LibreOffice, which was simply achieved. For some reason by default LO's UI was in dark mode so it had white text on a light-grey background with button art having white borders rather than black. I set it to CoLibre standard theme (it was set to CoLibre dark) and then it looked fine.
Conclusion of sorts
The vibe I'm getting is what I would expect from a Debian install: Needs polish. However let's say if your grandma needed a basic-use PC that isn't going to try and get you to store your soul in the cloud every 5 minutes and/or throw adverts at you and you have the inclination to change some options here and there to polish up the install a bit, I think it would work. The fact that it's running Linux and that the makers of the Pi 500 intend to continue production until something like 2031/2034 gives me confidence that long-term software support won't be an issue; had it been some other OS my concern would have been a scenario whereby an OS was produced and then abandoned a few years later.