Depends on the game I suppose. I find it easy to go back and forth between WoW and WAR, although LOTRO is a little tough due to there strange UI and UI artstyle, but I think that's what alts are for. If I feel disoriented due to different controls, I'll make an alt, play him for a bit till I get used to the game again than go back to the character I wanted to play or continue with the alt.

I tend to jump mmorpgs a lot through out the year, especially during dryspells like we've had lately. It is very hard to jump when there is brand new content like a new game or expansion, as those usually give at least 2 -3 months worth if not more of content to go through.

As opposed to content patches, which only last about a week or 2.