I spent a night there in 2006 before spending the next couple at Paradise Pier, and I was less than impressed. Bad plumbing in the bathroom, non-working ice machines and a couple of maintenance issues with the rooms (which were nice compared to some places, but would have been nicer if they didn't fall apart.) I suspect the specific issues I mentioned have probably been fixed, but I'd probably be hesitant to stay there again.
Walking distance to the parks is about 3/4 of a mile to the park gates via the Grand Californian lobby. If you've been walking all day that'll seem fairly long, but it's actually not that much farther away than some of the Harbor Boulevard hotels.