The only time's I've used the Peace Bridge and the Queenston-Lewiston bridges is the the Can-Am ride put together by the NFBC (Niagara Frontier Bike Club). We stayed to the right hand side (through Duty free) on the Buffalo side of the Peace Beidge and then rode across using the side-walk. Since we were expected the customs folks were waiting for us. On the way back to the USA we used the side entrance to the Queenston-Lewston bridge. The fun part is that you have to cross ALL the lanes of traffic that are right behind the toll barries (all the traffic coming from the USA) and then cross the few lanes of traffic going to the USA. Thanfully each ride was on a Sunday and it wasn't too busy, but crossing something like 12 lanes of traffic is a bit intimidating. The cool part is that we got right hand lane to our own use. Amazing how one time a car got behind us and honked.... the lane was CLOSED to cars, but apparently reading was not a skill the driver excelled at. We were allowed to pull around the trucks in-line and got through customs fast. Without having this be scheduled, I am not certain I'd dare use the Queenston-Lewiston bidge.
I personally have not ridden accross the Niagara Falls bridge, but that has a well used side walk as there is a constant stream of pedestrians crossing the border there.
For a nice Centry ride - Start at the Peace Bridge. Cross into Canada. Follow the road along the river to the Queenston-Lewiston Bridge. Cross over the the USA side. Follow the local loads along the river again to Old Fort Niagara. Go along the lake to Wilson. Then ride back to Lewiston, go up the escarpment. Follow the Robert Moses (now one side of the road is a bike path). Pass along Niagara Falls. Ride along the river through the Tonawandas. Follow the River all the way on the bike path back to the Peace bridge. For those that want a Metric Century, once back in the USA cut out the ride to Old Fort Niagara and Wilson and instead head straight back to Niagara Falls and the Tonawandas.
Happy multi-country riding,
André