The problem with taking decongestants or flying with blocked sinuses is not simply the present pain or the pain relief; it is the subsequent consequences.
In the climb, as the cabin pressure reduces there will be a buildup of pressure in both the sinus and the inner ear. Both should vent naturally into the nasal passages. Any inflamation or congestion will then reseal the passages - you will feel stuffed up etc.
When the aircraft descends the pain will first be noticeable in the ears as external pressure on the tympanic membrane will cause pain and ultimately it might burst. You will not necessarily the sinus as the sinus cavity is surrounded by bone.
Now a decongestant will undoubtedly open up the eustachian tubes and allow pressure equalisation and also in the sinus cavities. Unfortunately there is an excellent chance that the gunge that closed the tubes in the first place, an excellent design feature, will get into the sinus cavity and the inner ear leading to infection and potentially deafness.
The moral? Don't fly with blocked sinus or a sever cold!