The main problem with open grates is that the gap between the bars is often the right size to grab your wheel, which means you take a nasty spill (technically called a "diverting fall") to the pavement. It hurts. Watch for that stuff, and if the whole bridge is like that, it may be a good idea to step to the curb and walk that stretch.
I've got two different brands that I like... The "Take-a-Look" is probably the one that I like best, since it gives a little wider view behind you, and the mirror lenses are replacable if bad things happen. The other one that I like is the "Third Eye," which is a round lens, a little smaller field of view, and the plastic clip that clips it onto your glasses is prone to break if you treat it ham-handedly.2) Any rec's on a helmet mirror? I think I'd feel more comfortable knowing whats behind me - although I realize I'm much more likely to be killed by getting doored than I am by someone behind me, it would still be nice to have one.
Another skill to practice is learning to look over your shoulder to check behind you for overtaking traffic on either side. You might forget your mirror sometimes, but as long as your head is still attached to your shoulders, you can scan behind you ;-)