Agree, the cancellation policy is the biggest con. You may be able to put in a room request yourself, but some rental companies may stipulate that needs to be done by the member. You may need some patience in doing a rental as well-- unless you're doing a pre-booked rental, it can sometimes take a while to get an owner match for the rental you need, particularly if it's more than 7 months out.
Of course, it's typically substantial cost savings, so that's a big pro.
Not really a pro or a con, but some folks rent directly from members rather than through a third party company (there's a whole forum on this board for that).