I use it at home, because it's free. It has it's quirks, but for a free software suite, it is leaps and bounds ahead of some of the competition. I sometimes use it to make changes to work i've done at the office using MSO, and the compatibility is fine.
I don't use it enough to judge as to wether it would make an able replacement for MSO in a heavy usage environment though.
It has been said here (probably by Burnsy) that things like macros and massive excel formulae do not work well.
In other words Open Office is more than sufficient for the average home user but inadequate for many businesses.