I had a 721, purchased after they fixed the SW. And, it was great. Way better than any 5xx series DVR.
Yes, once it settled down I also really liked it and thought the UI was better than the current UI. However, the only thing that really made the 721 PITA was that it was NOT name based recording. Had been NBR, it would have been a really close to perfect dual tuner SD DVR (we use our TV2 dual mode for recording content to DVD while we still can watch TV 1 in HD and also distribute other TV2 channels for occasional independent use, so the 721's support for ONE TV--yes, it did have a Ch 3/4 output, but not the same as the multiple TV2 experience we have now--was also a bit of a negative).
I really liked the Weather banner. It was aesthetic, instant, and very usable, handy, and I used it EVERY DAY. One didn't have to go to that stupid Dish Home portal to wait just to check the weather, and I am a but nonplussed that Dish couldn't have the same feature with the subsequent DVR's. The only really big thing the then new 522 and subsequent 625 had over the 721, IMHO, was Name Based Recording and I thought the UI was a step DOWN from the 721.
Remember, the 721 also came with some games preloaded and FREE of charge. I did, in fact, play them on occasion and really do miss that somewhat. The TiVo's I have also have preloaded games and I admit to occasionally having fun with them. I know; for shame on me, but sometimes you just want to veg-out.