From playing with the X a few times in a store and with the ones my friends have, I can say the feel of the buttons changes considerably, depending on where you are.
In a nice air conditioned store, they fell loose, but take one out to the desert in 110 degree heat, and having been in direct sunlight for a few hours and they definitely do not feel as loose.
Plastic expands in the heat, so the buttons under most circumstances might appear to be loose, but if they warm up, they Will expand.
As for the feel in a store, they feel no cheaper than any other type of plastic push button I have played with. That includes my BBs, my GPS devices (although they have rubber covers over the buttons), my MOTO two-way radios, and even the MOTO satellite phone. I believe the buttons should last the normal life of the phone. I would hope, MOTO did some robotic tests on these buttons before releasing the X.
But that is only an opinion at this time.