FWIW, good SD, like a good/clean DVD or the somewhat rare clean SD broadcast, looks quite good through my Planar. But it's SD so it's still noticeably softer than Blu-ray.
As far as upconverting I haven't seen the Oppo, but the Planar is at least as good as my Pioneer 51FD.
But from what I can tell it's very hard to tell what to expect for SD reproduction on FP systems based on other people's descriptions. What I mean is I think some people just simply won't be happy with SD on anything bigger than a 27" tube, while others (like me) have accepted SD for what it is and can be reasonably happy.
Final FWIW, I don't think the Planar does anything "wrong" when it comes to SD, though I suppose it's possible that a being a DLP and having the high MFT/ANSI/etc the deficiencies are more apparent than they would be on something softer.