I put a diplexer between my H10 and rabbit ears (yes, rabbit ears). I live 15 miles from the nearest Baltimore tower and 22 from the nearest DC tower. Great signals. Right afterwards, I watched a game on ESPN2HD and thought it looked terrible, automatically blaming my diplexer. Well, it turns out that the game was SD after all. All the others look the same. NFLST looked great in HD, so, no, I don't think there's much appreciable drop off. And the US open on my rabbit ears? I have never ever ever ever enjoyed tennis like that (most definitely NOT a tennis fan) and was glued to the screen.
Oh, and why rabbit ears? For 10 bucks I get all 4 majors from both cities, plus all PBS, WB, UPN, and the majors' weather channels. The rabbit ears are nice to be able to move to maximize signal according to antennaweb.org.