I've stayed in the CP 4 times in the last year as IHG Spire, paying best discounted rate. I've had upgrades to the exec floor , and lounge access every time, and 3 out of the 4 I was upgraded to a suite.
The lounge is pretty small and minimal,and I tend to go downstairs for breakfast. As a previous poster said, the breakfast is basic. (But I find that a lot in lounges)

The pictures above are pre-refurbishment, and they are much nicer rooms now. I go to concerts in Glasgow, and it's my go-to hotel. Ocassionally , if the concerts are more central, I've booked places like Fraser Suites, Indigo, Rad. Blue, Park Inn, etc.

The bar can get busy after a concert, but I've never seen it packed out.