Interestingly enough, for a contrasting position, the Verge review calls out the OS for being poor at handling multitasking:

"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...