I tried an S21 Ultra back in the summer. Overall, its a powerhouse device that will handle anything you throw at it. However I ended up returning it because of the size and weight. It was big and heavy and just made it hard to use and carry.
I ended up replacing it with a Certified Renewed Unlocked S20+ directly from Samsung and am very happy with it. Its significantly lighter and slightly smaller making it far easier to handle. I went with the S20+ over the S21+ due to the extra RAM and higher resolution screen. I think it is a very solid device still with plenty of life left in it. Still handles everything I throw at it with ease and the battery life is great. Screen is fantastic and camera is very solid.
I have an S20 plus and still love it. Don't get caught up in all the hype. Upgrades anymore, year after year are very minimal. In some ways, the S21 series is actually a downgrade from an S20 plus.
Would be a good buy, yes. You can get it heavily discounted now and 12 Gigs of ram is fantastic. And with Samsung expanding their software support another year or two for it, it would definitely be a good buy right now.