One thing to keep in mind - that it can be difficult to tap the equity in your investment property. That's just the current lending environment, and maybe it will change ... but that could be for better or worse.
So when you make that extra principle payment on a loan, and then you wish you had those extra funds as cash-in-hand, don't be suprised that there might be lender qualifying that prevents you from getting at that extra money that is now equity in a property.
There was another thread that discussed paying off existing properties or buying more, and where I agreed with another poster who said to do both.
http://www.biggerpockets.com/forums/12/topics/34337-pay-off-properties-vs-purchase-more-properties
I still think you should do both - you just have to plan how much and how fast to pay off when you are in a buyer's market. Having your funds tied up and unavailable these days could result in you not making as much over the long run.