Do ALS patients suffer painful, suffocating death?

My view. Yes, many ALS patients suffer pain, but they rarely suffocate, as typically, when needed, a tracheostomy tube is placed. But demise can often be due to pneumonia.
Not with care. While ALS is ultimately a fatal disease, patients can be treated both to prolong life and to ease discomfort. For some patients wihen the breathing muscles are mildly affected a small machine with a mask called a bipap device can help a patient breath, mostly at night. With advanced disease when bipap is not feasible, etc meds like narcotics can ease air hunger. No one need feel suffocation ever.
Sometimes. An ALS person is more likely to die of pneumonia -- which a wise physician / family would not seek to treat. Very weak people cannot protect their lungs from bacterial infection by coughing and breathing deeply. This is far less frightening than death purely from weakness, which actually would be slow suffocation.