I think it's great. I think 25% is a little too low, but on the other hand, there's steams cut, the publisher/dev cut, and then the mod cut. I'm positive Valve will take 30% across the board like they do with everything else on steam which I'm okay with honestly.
Don't like the mods? Don't fuckin' buy 'em. But I honestly feel like this will lead to more games releasing dev tools which in turn will eventually lead to a much higher quality in modding games in general and people will have to step it up. It's a complete over-reaction to say this will destroy modding. Modding is going absolutely nowhere. Go donate on nexus if you really care about the creators getting paid, otherwise just shut the fuck up and don't buy mods? Like everything else in this world, vote with your money.
There is the current issue of people posting mods that they didn't make but I see that going away very fast. It's already happened, even.