Pro: you get access to some features such as mirror lock up that are otherwise not available on the 300D.
Con: you have to read the 10D manual for some features, and not all the 10D features work on the "hacked" 300D even when they appear to be available in the menus. This makes using the camera more complex.
I put the "Wasia" firmware on my 300D right after I purchased it. No problems observed. (Not that I've done as much photography with it as I wished to. But that's another story.)
Would you use one or more of the hacked features? If not, easy answer. If you would, you'll have to decide.