I got one of these as well. Timely, because I have a trip to Hawaii booked on UA in November (ATL-HNL $533!). It would be nice to be able to access the HNL lounge before my late-night return. My connections are too brief for visits, but if something goes amiss, it would be great to have access.
It seems like you don't even have to charge anything to the card. Just get approved, and you get your membership card in 6 weeks. I'm very tempted to go ahead and take the plunge, since it basically seems like completely free lounge access. The only downside I can see is that it will count against the 5/24 rule--I'm at 4/5 right now, but two will drop off March 2020. It's possible I could end up on some additional UA trips in the coming year as well, as they seem to have entered a period of competitive fares out of ATL (they've done that before).
Anything I'm overlooking?