Everyone is different. I enjoy the trip planning and see reservations as a part of that process.
If you have been to Buena Vista you know that there are several good choices. Arrowhead Point north of town and KOA on the south are my first choices, but there are Snowy Peaks and Mt. Princeton in town. Out a ways is Chalk Creek (which used to be Browns RV). Lots of Forest Service campgrounds around.
In Colorado Springs our favorite by far is Cheyenne Mountain State Park and there reservations are a necessity and have to be made far in advance to have any chance of staying there.
In Estes Park we have always stayed at Spruce Lake, but that is because we have friends that always want to meet there. There are many other choices inside and outside the Park, depending on what services you need. We have always felt that reservations are a necessity there, especially in September when the Elk are active.
Just some thoughts.