For the first six months or so, I kept all my e-books on the Kindle. But when my collection grew to over 1,000 (I have almost 7,000 now!), I found that the device had slowed down too much and had intermittent problems.
Now I keep just a handful of reference-type nonfiction, a few "active content" games/utilities, and one or two fiction books on the Kindle. I delete the books after they're read. That works much better for me.