Imitrex will stop my migraines in a short amount of time. It is a godsend. Stops the pain and also stops the nausea.
I do, however, have aftereffects: usually back pain later that day. Also, grogginess and fatigue. It is still worth taking Imitrex, though.
For me, it is best to take it as early as I realize the headache is a migraine, instead of waiting too long til the headache is going strong; harder to stop the pain once it has reached that point.