I've done the Malahat once, but never ridden all the way to Nanaimo. The 'Hat has steep sections mixed with level sections, and climbs to about 365 meters (about 1200ft). In some places the road is wide, smooth, and has good shoulders for cycling; in others it is narrow with rough shoulders. The views, however, are gorgeous! You can bypass the 'Hat with a short and pretty ferry ride.
After the Malahat, the route is rolling terrain with long gentle grades and descents, with reasonable shoulders for riding in most places. Traffic can be frantic at times especially on the older, narrower sections, and there are usually road repairs/upgrades along the way. There are side routes as an alternate way to Nanaimo, but they add significant distance and time to the trip.
A brief discussion recently on a local board:
http://groups.yahoo.com/group/gvcc-members/message/4249

Please PM or email me if you need more info!

~Sherry.