Attaboy! I've got three, and it's kinda like having a family around.
But, really, if your PRS-505 does the job for you, keep it as long as possible. It's still a pricey kind of hardware which one usually doesn't replace every other week.
And I do think there will be a line of advanced (and quite different) reading devices availabe in about 3-5 years, so it's soon enough thinking about a replacement by then.