In general, I find the 5MCC to be very useful for quick reference and for devising a plan for diagnosis and treatment, but it is not a definitive reference. Harrison's, though complete, is extremely difficult to "scan," and so I generally avoid it unless I have a topic that is difficult to find elsewhere (which there are very few). I find Outlines in Clinical Medicine to be a very definitive text with tons of information that is much more readily searched and scanned than Harrison's. It is available for the PDA and probably worth purchasing. I would recommend Ferri's Guide to the Clinical Patient (paperback or PDA) over the Washington Manual because it includes a section on differential diagnosis, many of the labs, and just seems more likely to have the information for which I'm looking.
The Merck Manual (text or PDA) is also a much more readily scanned and searched "step-down" Harrison's. Though less complete, the Merck Manual will often have some more details that may not appear in the 5MCC or Outlines in Clinical Medicine.
My recommendations on PDA reference sofware to own:
5-Minute Clinical Consult, Outlines in Clinical Medicine, Ferri's Guide, and Taber's (or Stedman's) from Skyscape
Merck Manual from HandHeldMed
Grant🙂