Probably Less than 50%
in the northeast corridor, they're not bad, and at least as reliable as the airlines.
However, on routes headed for Chicago, you are at the mercy of the freight railroads that control the tracks over which Amtrak operates (I think Amtrak owns the Chicago-Detroit line--the old Michigan Central R.R., but that is an exception to the rule).
I take Amtrak 2-3 times a year Washington, DC to South Bend or Elkhart over trains 29-30, the Capitol Limited. My experiences in 2006 were that Amtrak runs pretty close to schedule (and the schedule is padded) between Washington and Pittsburgh over tracks owned by CSX, but at Pittsburgh and beyond switches over to tracks owned by Norfolk Southern (Pittsburgh-Alliance, Alliance-Cleveland and Cleveland-Toledo-South Bend-Chicago), and is really at the mercy of NS. (The Lake Shore Limited, which runs on the historic NY Central lines now owned by NS Cleveland-Erie-Buffalo-Syracuse-Albany-Boston/NYC, has some horrific delays). Particularly troublesome are the areas around the Toledo station, and the large freight yard west of the Elkhart station. I have been on-time as far west as Goshen (10 miles southeast of Elkhart), only to get stuck at a red block on the north outskirts of Goshen for 45 minutes, even though the way is at least double tracked.
I tried to go Elkhart-Chicago last October--the train was scheduled to leave Elkhart at 7:04 a.m. EDT and arrive in Union Station at 8:30 a.m. CDT. The train pulled into Elkhart at 8:15 a.m., detrained about 20 passengers, took on 3 passengers (including me), and then set out--it made it over to the South Bend station in 20 minutes, and was running at good speed until we got over to LaPorte (next county seat west of SOB), when we got tangled in a freight train congestion mess which involved single tracking around a stalled freight, which lasted until we got past Gary. We didn't get to Union Station until 10:45 CDT--in other words, about 105 miles of train riding in 3-1/2 hours--not too good when you consider that the speed limit is 79 mph for most of the route.