Due to the chipset used on the Sansa, the battery life is definitely lower using Rockbox compared to original OF.
Also USB charging, from what I hear is much slower in Dropbox. Some threads mention that you should always use OF for charging as well as USB transfers, as USB transfer is not quite implemented in Rockbox.
Thee are definitely some Nit Picks. I am very satisfied with Rockbox on the E200 series, and Rockbox alone has exteneded the useful life of my Sansa immeasurably, with the SDHC support.
Not to mention Boot up time is definietly much faster in Rockbox than OF. I just wish Sandisk would release some documentation to the Rockbox team now that the E200 has been "officially" retired. With the proper documentation, I am sure that the Rockbox team could make the battery life better. But it's definitely not a problem that really cripples anyone.