Downsides?

The downside would be it drags the loan back out 15-30 years whatever you choose.  

This has been a big downside being so far away.

Building wealth via money in your mattress and cd's was pretty good in 2007-2008Diversifying your portfolio is used to limit downside.

Is there any down side to this?

It's free if you don't expedite the payment, the only downside is it may take about 5 to 10 days to receive your money.Rentec Direct, is a full software but you can accept rent online for a monthly fee plus set up fee.

Since it is a loan there are no down sides for tax purposes and the rates are very low at this time. 

HELOCs are typically higher interest (downside), but are revolving so they can be tapped over and over (until duration of term is over) assuming you pay balance down prior to re-tapping it.

If these properties aren't appreciating or your cost/rent estimates are wrong, this gets ugly very quickly....I'm a pretty conservative person, and a newbie, but I see a lot of downside risk in those numbers.

I don't have much time to write about the downsides of employer-sponsored 401K's, but go to 401kaos.com and download/read the free book for more insight.