I have not been in there as I usually ride Business Class so already have the ML and priority boarding. However I've walked by it and it looks VERY nice. If you were traveling coach and had a several hour layover I would certainly pay the $20. Or if you were local and a Metra commuter I can see paying the $20 to go wait in there if there was some kind of system shut down...much better than the craziness in Union Station!
I was riding in BC to get the access to the ML. After some research, I actually found that it is cheaper to ride coach on the Pere Marquette to and from Chicago and then pony up the $20 for the Legacy Lounge than to ride Business Class round trip. The snacks and beverages are better and the attendants are friendlier in the LL. In addition, you can bring outside food into the LL where you cannot in the ML (unless you sit and eat in the entry area). It's just something for you to consider being that you travel to Chicago often.
That would work fine on the double deckers since coach has the same roominess. But since I am always working on the train, the single level cars are just simply too tight for me, particularly if the person in front of me would like to recline!
I do ride the Pere Marquette inbound in coach sometimes in the winter since that's often less crowded than business class and I can get 2 seats! More a matter of space than anything in my case!