Hi, I have a 2015 late 27" iMac. I maxed it out at the time (highest-end graphics card, upgraded the RAM myself, 512 GB SSD) and it's still a very decent computer today. I've compared notes with someone who had the lower-end graphics card and their system struggled a bit with some current photo-editing software, whereas mine did not.
I don't know that I'd buy one today, though, unless your budget couldn't stretch to a newer one. There are a few reasons for this:
1) It's the last Mac system to use Thunderbolt 2. Thunderbolt accessories really took off with Thunderbolt 3, which is also backward-compatible with Thunderbolt 4. If you want to use any of these things with the 2015 iMac, you'll need to get a Thunderbolt hub, and those are quite pricey. The biggest benefit of Thunderbolt 3 and up is upgradability. You can have a SSD running over Thunderbolt that operates essentially as fast as an internal SSD would. You can run external graphics setups, thereby further extending the life of your machine.
2) As you and another noted, it is now the oldest system to be supported by the next version of MacOS. Apple doesn't always drop systems from support with each version release, but it's close to being cut. If Monterey isn't the last version it'll support, it's very likely that the version after will be. Granted, you can still use a computer even if it's a few versions behind the latest and greatest, but then you start missing out on features.
3) Lack of support may occur even faster than usual, as Apple is moving away from Intel-based systems.
The pink edges thing... initially most of us thought it was a sign of impending display failure. Now it's thought to represent aging of some adhesive tape that Apple used. It can be annoying, but in most cases I'd say that unless you're specifically looking for it, it won't bother you too much. This may vary depending on what you or your family are trying to accomplish.
If you're looking to have a computer for the next 2-3 years, it's not a bad option. If you're thinking 5+ years, I'd recommend at least the 2017, or later if you can. The
Apple refurbished store doesn't have any 2015 or 2017 27" iMacs in stock at present; for 27" they only have 2020, and they start around $1,500. If you go down to 21.5" systems, though, they have older ones available... just be aware of the limitations (mainly, inability to upgrade your own RAM on the smaller systems).
