I'd say it's uncommon in our group, at least until higher levels. In an epic game that went to 18th level, 3 of the 7 PCs multi-classed after 11th level. Two of those were simple 1 level dips to fill out a weakness (but the players did a great job of RPing the addition). The other one was a planned setup where the Rogue went to 11th level, then switched to Champion Fighter.
In all the other games we've played, most of which ended at/near level 10, maybe 1 out of every 10 characters would multi-class. All of those were single level dips, usually at level 2 or 6 (I guess my group thinks those levels have the weakest benefit). I think the low level benefits for single class characters are strong enough to keep people from wanting to multi-class until higher level (where delaying a benefit for 1 level isn't as big a deal).