Per George Geczy on the BG Forums:
The "Kills" number (shown on the right side of the unit popup information) is really just for interest sake, and does not directly affect experience by itself. The true "Experience" factor is the bar next to the medal icon at the top right of the unit popup (it also appears in a number of other places in the User Interface panels, when looking at units).
Usually the bar next to the icon is 'zero', but when a unit starts to gain experience a green bar will start to grow from there. If you mouse over the green bar, it will report an experience rating as a number.
As a very general rule of thumb, a unit with experience 100 will be about twice as strong as an equal unit with experience 0. Now, there are many things that can affect that result, including efficiency, morale, alert level, and of course the old standby modifiers like terrain, but as a very general rule this should work.
Also note that there is a diminishing returns factor at play here - a unit with experience 200 will only be about 2 1/2 times as strong as a 0 experience unit, etc.
In one game I somehow ended up with some strike fighters that had reached experience 300 or so, and they were a very formadable set of units - I could strike AntiAir stacks head on and come out with few scratches, it took an interceptor attack outnumbering me 2:1 to finally get my fighters out of action back in the repair hangars.