Last time I was there BA and VS and others used the Club America lounge, airside. There are two parts, the business and first lounges - the first doesn't have much in the way of windows while the business area does.

The lounge is spacious and comfortable but dull and poorly catered. As others have said, soup and very boring sandwiches. Drinks can be plentiful and the staff are not unpleasant. There are showers, but not all airlines allow their pax to use these, if I recall correctly, VS UC pax get the first lounge but not free showers, whereas BA first do.

As with anything to do with MIA, spend as little time as possible there.