I'm not Jim, but I can shed some light on your situation. I live in a rowhouse not far from 20th and Pine and have had the same difficulty receiving clear over-the-air analog signals, even though I have a large rooftop antenna and a rotor. The multipath (signals bouncing off of the tall buildings) caused ghosting even on the best reception days.
After I upgraded my TV to an HDTV-ready monitor, I ordered a set-top box because I wanted to see the HDTV transmissions (and I've never been a fan of cable). This tuner is attached to the same rooftop antenna that was not able to overcome the messy analog signal we get in Center City.
I am really glad that I made the transition to digital. I still cannot receive WCAU for some reason (but I fully expect that I will be able to get this signal clearly at the end of January when they will move to their new broadcast tower and increase their signal strength). I get ALL of the local channels broadcasting coming from Northeast Philly strongly. With my rotor, if I point the direction to the East, I can also receive several New Jersey stations. I have had no luck receiving stations from New York City.
The HDTV broadcasts are wonderful, but to my pleasant surprise, even the "standard definition" digital broadcasts are MUCH MUCH clearer than I had ever imagined. I don't have cable or satellite myself, but I have seen it in other homes, and the digital OTA broadcasts are notably sharper than the best cable or satellite signals I have seen.
There is NO possibility of "ghosting" (the usual manifestation of multipath interference) with digital broadcasts. If there is something interfering with your signal, then the worst outcome is that your set-top box will not be able to lock into a signal at all. That has happened maybe two or three times since I installed the digital tuner, and sometimes I suspect the digital broadcast was maybe down temporarily (e.g., for maintenance on the broadcast tower as reported on the early pages of this thread). Other times, adjusting the antenna slightly has brought in the signal. Of course, I can always tune in to the analog broadcast as a backup.
For a digital signal, a "bad" or weak signal shows up as pixellation (I think I spelled that right) of your video image. I have occasionally seen that, but it is very occasionally and not often enough to be bothersome. I've reported here about a "flicker" in the WB17 HD signal, but you have probably read the earlier posts on that problem already.
Even if you don't have an HDTV-ready set, I recommend that it is very worthwhile getting a digital tuner if you don't want to go with cable or satellite. My tuner, for instance, has outputs for "regular TVs" such as component, S-video cable, or even composite. If you already have a rooftop antenna, a digital tuner may be the next logical step for you, in order to get a nice sharp over-the-air signal. Then you will be well positioned when you are ready to upgrade your TV set to an HDTV model. Some online dealers (such as Crutchfield.com) will even give you a 30-day no-questions-asked return.
Good luck and let us know what you end up doing!