Replied to the iphone/ android thread back in March, when I traded my S9 for an unlocked S21 / 256GB through Samsung... Trade-in worked out well. I like the phone, and had no issues getting it setup (was already on Verizon, so, was able to use my old sim). Only thing I noticed was that the antenna/ radio in the S21 doesn't seem to be as strong as the S9. Especially inside my house. Wifi calling mitigates some of that, and, there is a free app you can d/l that allows you to select LTE bands only. If you're not in a good 5G area the phone wants to grab the 5G tower, even w/ a poor signal.
I eventually switched over to a 15GB prepaid Verizon plan w/ 5G, and swapped out my old sim for a new 5G-capable one (bonus- seems like it's helped my coverage). Verizon will give you one for free- either at a corporate store, or, by mail.
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Also, a quick note from that thread regarding trading in w/ Samsung- bunch of posts about this on reddit as well. I'd take pics of your phone, screen, and boot up once you backup/ wipe it, jic. Fortunately, I didn't have any problems trading in, but others seem to have experienced major problems.
=> If you trade an old phone in, make sure you follow the Samsung directions to the letter--- backing up your data, then logging out of Google/ Samsung accounts, before factory resetting from the Settings menu (not recovery). Seems a fair number of people were getting jammed up on the trade-in, as their phones weren't actually wiped correctly, and the techs couldn't validate due to Factory Reset Protection.