I've worked for four software companies in my career and it's only in the most recent employment that I've cottoned on to just how restrictive clauses like this are.
Simply put by agreeing to these clauses you hand over ALL your intellectual property rights to the company for the duration of your employment.
That website you created for a friends business? Belongs to your employer.
That database you wrote to store your CD collection in? Belongs to your employer.
That iPhone game your wrote? Belongs to your employer.
The question of them enforcing it is largely irrelevant - they own the IP either way.
This is the reason I now remove any clauses like this from any contracts before I sign them.
Simply put by agreeing to these clauses you hand over ALL your intellectual property rights to the company for the duration of your employment.
That website you created for a friends business? Belongs to your employer.
That database you wrote to store your CD collection in? Belongs to your employer.
That iPhone game your wrote? Belongs to your employer.
The question of them enforcing it is largely irrelevant - they own the IP either way.
This is the reason I now remove any clauses like this from any contracts before I sign them.