PDF piano sheet music hasn't worked too well for me because the page breaks tend to fall in the wrong places (i.e. the treble part will display at the bottom of one page, and the bass at the top of the next). You'll end up having to do a lot of zooming and panning manually, which doesn't lend itself to quick page turns. If you transform the sheet music so that the page size is exactly the size of the reader it works better but this is a lot of work.
Programming books fare better but the diagrams often end up too tiny (although it's easy to zoom in). I find it easier to read technical books in landscape mode, although there is more scrolling, most tables and diagrams fit better that way. The programming books that I've had real trouble with are the 'Head First' series because they tend to have a lot callouts and things going on in the margins, as well as a layout that assumes you can see both pages at once. Also books with large blocks of code are frustrating because you can't see the entire class/function on the one page.
I prefer to use my iPad to read both of these types of content and use the 650 for reading novels and papers (which are more text-based).
P.S. Welcome to MobileRead!