How many records can Zoho CRM comfortably handle?

How many records can Zoho CRM comfortably handle?

On the website, it states that Zoho CRM can handle unlimited records on the Enterprise plan. My client has 500,000 customers. I'm curious to hear people who have worked with data sets as large as this. How did Zoho CRM perform? Did it cause a slow down? 

If a Zoho rep replies to this, I'd love them to connect me with an existing customer who has a database this large as it will have a bit more credibility coming from them:P