The principles here are simple enough. The practicalities are nuanced.
I'm generally on the side of user-modifiable Web content once it's reached the user's computer. Google maintains that's all it's doing here, and that users have ample choice about whether to download the toolbar or not.
Fair enough, but not sufficient. Google commands a special position, and the Toolbar default settings -- which are what most people will use -- are going to give he search company too much influence. This is a natural move for Google, but it should re-emphasize to everyone that the company's motives are the standard ones: looking out for its own interests, period.
Site publishers have never had the absolute ability to determine what readers see. But what Google is doing here goes beyond giving users a way to, say, resize fonts or block popup ads. It's using other people's work for its own -- and its partners' -- commercial purposes in a way that alters the content.
The alterations aren't that big a deal, not today. But it's inevitable that they will grow. This is too powerful a tool not to be used in more expansive ways.
Google still offers me value. But so do the alternatives, and I'm increasingly seeking those out.