i bicycle this path frequently, generally from around 24th down to 5th. when i am less of a weakling and can handle biking back up the big hill i will start further north, but i'm not there yet.
in the downtown section you go under street crossings, which is nice because you avoid traffic but not-so-nice because there are often homeless people sleeping/living/lurking there.
no problem on a fast bike, but not very pleasant for an afternoon stroll.
on the bright side, when you're walking in the nicer part there are several signs that describe the historical importance of various sections of the trail. i learned a lot!