The heaviest things I've carried on my M12 are three massive text books (easily 15 pounds) and, on another occasion, a 24-pack of beer. I'm usually riding with something up front as I find it extremely convenient to have a basket. The only time I find having a lot of weight up front as a burden is when the bike is at a stand-still and I'm maneuvering it around because the fork will quickly turn and knock the handlebars into the toptube.....not really a big deal when riding around because I get used to the different steering feel after a minute or two.

Here are a couple of pics of my bike with an M12 and a basket. I really like the M12 because I've attached a blinky under the basket and some reflective strips....good stuff for maintaining visibility when riding at night and I don't have to sacrifice any handlebar real estate.




And here are a couple of pics of a bike I found while in Santa Cruz a couple of years back. This bike has the big basket on it with a different Nitto rack, however I'd wager that it can handle a lot of weight.