Interestingly enough, the Verge review calls out the OS for being poor at handling multitasking, so it sounds like they don't even have much of a leg to stand on there:

"I don't feel BlackBerry 10 deals with multitasking or notifications as effectively as other platforms do (most notably Android), but it's not a total strikeout."

http://www.theverge.com/2013/1/30/3929760/blackberry-z10-rev...