We spent some solid time (and lawyer fees) getting this right, after inheriting a draconian agreement from our long-ago corporate parent. Now you own anything you do on your own time, so long as isn't directly relevant to our business.
By the way this didn't stand in the way of investment at all, as some commentators here suggest.
Define "directly relevant". Sometimes that is the extent of the clause.
Jury: well its computer software, and your business states right there in the business code classification that your company is a software company, so we say its owned by the business.
So, no, if I invent a new elephant feeder, it'll be pretty clear its not directly relevant to my employer. But since I spend all of my spare time coding, I'd need "directly relevant" spelled out pretty thoroughly.
There's a disclosure process; if you're concerned someone might in future define your project as relevant, you submit a description of what you're doing and the company has a limited time to claim the project is indeed relevant - if it doesn't do so, then it gives up the right and the project is yours.
(Disclaimer: I'm the CTO not a lawyer, so this is just my opinion and doesn't represent the official legal position of my company.)
By the way this didn't stand in the way of investment at all, as some commentators here suggest.