I have a large group of players, some of whom come all the time when we game and some who don't (sometimes due to work, etc). So level disparities develop. I prefer a game where everyone's the same level, but with a group like I've got (and dealing with absent players by having their pcs fade out into the background) it simply doesn't happen that way.
I don't think we have significant difficulties with it; I try to give everyone their own stuff to do, subplots, etc.
In my low-level game, the high level guy just hit 8th and the low-level guys are 4th (two of them, though I'd bet one is going to level when I award xp before we play today). One of the fourth level guys often misses sessions, and often has to leave early (he's got a baby and more real life commitments than most of us), so he misses out on a lot of xp. The other one has missed quite a few of the 'big' sessions and has died a lot- so he's had a little trouble progressing in levels. (He just died again a couple of games ago, in fact!)