I was diagnosed with Ulcerative Colitis at age 22. It's generally accepted that people suffering from Crohn's, colitis, IBD, etc should avoid NSAIDs and COX-2 inhibitors as they can lead to flare ups of their disease. So, it's not that I absolutely can't take them without dire consequences, I just can't take them regularly, or for very long (like a day or two MAX). When my doctor told me this, I shrugged it off and just avoided them until I sprained my ankle so bad they were afraid I tore ligaments. I took Ibuprofin for 4 days when I was trying to avoid narcotic use and get some work done and I wound up with the closest thing to a flare up I've had since I went on Remicade years ago. So yeah, I won't be doing that again, but I'd pop a couple once every so often if my back was extremely sore or something like that.
The upshot is that I am really aware of how Ibuprofin affects me and I can feel it kick in and relieve pain the same way someone who could take it regularly might feel about an opioid.