It depends on your station. We have a couple of stations in our market that have 3 channels total and the SD channel looks terrible. Plus, having the other 2 channels up takes away from the HD bandwidth. Like last night I tried watching Angel and our WB digital is still upconverting the Analog signal and I flipped back and forth between the Dish local feed and the local OTA WB and they both looked awful.
Like the computer saying "Garbage in, garbage out".
It also depends on the size of your TV, if I watch the SD downconverted on my 27" TV then it looks great, but watching it in HD on my projector at 110", you see every little flaw. But, when it's good, it's awesome, when it's not, it stinks.