Was there a way to pay miles for the Presidents Club in the past? If not, this card was the cheapest way to get in given that you are going to have another airline card in particular. That was my calculation (well, and I got the first year free... but I was close to pulling the trigger for the thing anyway when I got the offer...).
So, if you can buy a United Club membership with miles in the future, this card is likely worthless in any case. If you are going to have to pay cash, the question will be whether card fee for the alternate (mileage plus explorer advertised on the side panel right now for example...) + membership cost in cash > presidents card fee. If so, the card remains economically worthwhile unless you plunk down for lifetime or something. If not, the card will be worthless.