Ive tried load balancing in a previous life, it does work if you have equipment in the exchange but still experienced poor results and some very odd routing patterns, at that time we used Q-Balancers from Xrio...urghh the nightmares.
The only real implementation which worked properly was using Cisco GLBP - Gateway Load Balancing Protocol but this requires two routers to implement.
Cant really comment for using multiple ADSL WAN connections on a single box as ive never tried it but i have used multiple point to point Serial lines using CEF on a single router which worked just fine.
Ill be honest, what are you after, throughput? Resilience? Policy Based Routing? Because personally i wouldn't recommend it. Were in the process at the moment of moving a customer with seven sites all with bonded adsl max connections onto a fully meshed mpls network much more robust.