Normally I would agree with the above poster...
However...
Mountain Lion DP3 is absolutely safe to put on personal Macs (mileage may vary).
I have it installed on my iMac and Macbook Pro.
Both work flawlessly. I have about 2 dozen pieces of software installed and all work well under ML, with many companies already offering compatible updates.
Mountain Lion DP3 is the most polished preview I have ever seen. Actually see improved speed on my machines. Safari is much faster. I could just imagine how much better the final release will be.