Depends on the alternatives, as Seeline says.
I always thought that primaries were much of a muchness. Transferred schools and have had real trouble motivating eldest ds with regards to school since switching primaries. It's been quite depressing.
Also, I now know the difference between what's on offer through a "good" primary as opposed to a less"good" one. And that makes watching youngest's progress through school quite painful to watch.
Lastly, I know the school isn't up to it academically, which is a pita because there is secondary selection around here and it means that the children at the current primary start the race with a big disadvantage.
There again, this is London and the differences between "good" and "not so good" are quite big. That may not be the case near you.
Also, what is "good" to you? It's worth giving some thought to that. I realised, after coming on mumsnet, that actually I cared more about ethos (I prefer liberal schools) than academic stuff (though I care about academic stuff quite a bit too), and what I considered a "good" school was quite specific.
There you go, just one opinion.
Can you not move and keep them at the same school?