The plant with the 2" pink bowl-shaped flowers in the front of this picture is Oenothera speciosa, the Showy Evening Primrose. I like this plant a lot. It is a native with lovely flowers. I realize that it has a tendency to run rampant, particularly in rich soils. My response to this is, "don't use it in rich perennial borders." That should solve that problem. What it is good for is weaving its way through informal mixed borders. In this particular bed, it replaces the Spotted hawkweed (when it stops flowering) as a binder, giving coherence and beauty to a very diverse planting.
Fewer inputs are better; the less we have to water and fertilize the better. As the population grows, water will become more problematic, both more expensive and scarcer, Dry Borders will become more important elements in the garden. This plant is an invaluable element in dry borders. It stays low, meanders enticingly, flowers dependably and heavily, clashes with nothing, and is pest and disease free.
In spite of all this, Showy Evening Primrose doesn't get a lot of respect in the horticultural literature, but it does receive a certain amount of affection. Ignorance leads to fear and fear leads to the dark side. If you don't know what you are doing, and you plant this wonderful plant in a fertile, well-maintained perennial border, it will take over. You would rightly fear it. But if you know the plant and you use it in the right soil, without supplemental watering, or fertilizing it will provide glorious strands of color over a fairly long period. I guess the flip side of "every plant can be a weed if its in the wrong place", would be that every weed can be a valuable addition to the garden if its used in the right way. Well, maybe not every plant, but definitely this one.