CBT - Should I enable globally?

We have over 300 VMs running VSphere 4.1 on NetApp storage. I have read numerous post about the benefits of enabling CBT and being able to track change rates of our VMs would be a huge benefit. What are the down sides of enabling CBT for all VMs? Are there any special things I should look at or do before enabling this? Are there disk I/O or other implications?

Thanks!