I've hearing on slashdot about these open source readers for some time, but only recently did I experience one. I had a 300MB pdf that Adobe Reader just wouldn't open at all. A day or so of reading forums and updating components and I finally got it to open the file.... takes about 5 minutes and lags whenever I try to scroll. So I downloaded Foxit (after reading about it on /., and I'll never switch back. It opens the scene in about 2 seconds, and scrolls nicely. (Not that the file DID open originally in photoshop *and* illustrator, but not the native application that is supposed to read the file.
I don't know what adobe did to reader, but it's unusable nowadays and frankly I'm done tinkering with it. It's a bloated POS. It's sort of how I feel about all of Adobe's software anymore, with the exception of lightroom.