Not a complete switch but i sold my most expensive Pentax glass to fund a u4/3 incursion (I kept my K7, K2, MZ5n and Pentax manual glass). Was I disappointed with Pentax? No!
My change was simply a question of needs, the Pentax gear was way to much heavy for travelling, (Pentax K7 + 16-50 + 1 or 2 lenses and you get a 2Kg bag), with u4/3 I get a relatively cheap system that delivers quality images on par with my K7 (probably a bit better on high ISO) but weights much less, Olympus E-PL5 (325gr) + 14/2.5 (55gr) + 20/1.7 (100gr) + 45/1.8 (116gr) that's less then 600gr for a great basic prime KIT, if you prefer zooms you have nice alternatives on u4/3 .
I kept my old manual glass for shooting film and keep my K7 running, the m28/3.5 is such a joy to use