1. Is your cement floor suitable for epoxy? Not every floor is, so review whether or not your floor has water issues, oil issues, too smooth cement, ground in 'tire marks, crumbling cement, etc. The following are some ways of knowing whether or not your floor is suitable for an epoxy garage floor paint:
The first problem to look for that will make garage floor paint not work, is if you sometimes see tiny crystals or white powder on your floor; these crystals or powder are an indication of moisture migrating through the concrete, and will often pop off any kind of coating you apply, regardless of the amount of prep. So, if this is the case for you, look for an alternative solution such as a garage floor mat. You can test your garage floor for this problem by putting a rubber mat on the floor. If water collects between it and the floor there is a moisture problem and any floor paint will probably come off.
Another indication that epoxy garage floor paint won't work is if the floor is already painted and the paint is peeling off; in this case most likely any epoxy you apply will also peel off.
If the floor is damp, the air musty, etc. the epoxy floor paints will probably not bond very well, and thus will not last long or look nice.
If your concrete has been treated with some sort of cement sealer (typically a waxy or silicon based sealer) no paint will stick to it, so if you don't know, try a test area out before spending a lot to paint the whole floor.
2. Would an alternative epoxy solution work better for you? For example, water based floor epoxy is a good option. It is thin, more fool proof, cheaper, maintains most of the cements anti-slip texture. Another options is a thicker solvent free floor epoxy, this type hides many flaws, is thick and glossy. Both are functional but the solvent free Industrial Floor Epoxy will look slick and slippery, while the water based epoxy will look just like paint.
3. The next question you should ask yourself related to garage floor paint is where you should prime or not prime? You should prime if you are going to use either the water based floor epoxy or solvent thinned epoxy (can be the industrial floor epoxy or several other epoxies). You will want to prime in order to get better adhesion especially if moisture might be an issue in your garage, or if your concrete is dusty and weak, or if it is really new, and the concrete is really polished smooth so there is not much surface profile for the epoxy to stick to.

If you use garage floor paint you want to make sure you properly prepare the surface. If you don't get the surface correctly prepared the coatings will not stick well, and your garage floor paint will come up. Occasionally a garage surface is such that no amount of prep will overcome the problem. Unfortunately, you cannot tell if you've done enough prep until after the fact, so better safe than sorry, when in doubt, over prep.