I have not heard anything good about the Upenn postbacc program other than the name, which is not a trivial thing. I have not heard a bad thing about the USC postbacc program except the cost. I completed the USC postbacc (they greatly discourage it, but it can be done in 1 yr if you are persistent) and loved it. I had 0 science background, unless you count HS Bio and Chem, and was able to do fine (mostly A's) in all the classes and the MCAT (34Q) with a ton of effort. The professors are excellent, except for Physics depending on who you get. They don't do a ton of hand-holding, but you do get priority registration, a strong letter from the post-bacc director who is a senior science faculty member, seminars on applying, MCAT prep etc, and professors love the postbaccs and are more than happy to write strong LORs if you take the time to get to know them. There are plenty of research opportunities as well either with basic science profs or at Keck. The postbaccs also have a strong community with parties, trips to bars, etc. They also pay for an MCAT review class, which is nice. The USC name is decent, and the postbacc program seemed to be well-regarded, or at least known, at all the schools I interviewed at. A good # of USC postbaccs have placed at UCSF & UCLA, as well as UW, Mich, Cornell, Vandy, Pitt, all over the place really (as well as Keck of course). There is no linkage with Keck whatsoever, many postbaccs don't even get interviews there. The published med school acceptance rate is 75%, but I think this includes people who drop out of the program before finishing. In recent years the actual acceptance rate has been closer to 90% I believe.
In summary, USC postbacc is expensive but worth the price if only for the name, guarantee of getting classes (with quality profs) when you need them, and ability to finish in 1 yr if you are willing to. But I also agree with the previous poster. If you are moving (I was local), look at Harvard Extension. It is a 3rd the price and the program is I think at least as well-regarded as USC. And have you looked into the linkage postbaccs, they cost the same as USC but you can be starting med school in 1 yr! Tough to get into these though and it's prob too late for this fall. Please PM me if you want to talk more about SC's postbacc program.