From my experience on the Cata beta and what I have heard other people telling me, while still viable the stats on these heirlooms have reduced effectiveness in terms of stats and scaling. Item levels and the overall balance of leveling gear have risen meaning that many quest greens and blues you find actually out stat your heirlooms until you get around two levels above them. A small but annoying change in the expansion I suppose, adding a little salt to the wound is the fact that alot of the Cata leveling gear as all new models, where of course your wrath heirlooms still have the old vanilla models.
A little annoying yes, but despite that the 20% exp bonus still makes them beat out any leveling gear you might find, that is until you reach level 78. At level 78 you will not be in Northrend, you will be heading straight to either Vash'jir or MT. Hyjal where NPC's immediately start handing out ilevel 284-300 greens. While technically your Wrath heirlooms do scale to level 80, they only scale up to the equivalent of ilevel 200 blues from early wrath. At this point, for these last two levels you have to decide if you either want the 20% exp bonus, or you want the vastly superior stats of Cataclysm quest gear.
So draw your own conclusions, I just thought I would drag this out there.