They're great if you are A MESSENGER.

I put my messenger bag strap across my body. E.g the messenger bag strap is on my right shoulder while the bag is on my left hip. I only experience shoulder strain when I put the strap and bag on the same side.

I love the timbuk 2 messenger bike, which I use to shove my work clothes in when I commute to my office. It holds a ton of stuff. In addition to a shoulder strap, it also has a body strap so that your bag isn't swinging wildly as your're riding about.

If you're not a bike commuter, though, I'd suggest getting a nicer, more professional looking bag. I'm an attorney, and would never take a timbuk 2 into a courtroom or business meeting.

Gotta agree, if you aren't an emo kid just get a backpack... Messanger bags don't look any more professional and they are way more of a pain to carry around. I just bought one that I was hoping would look more 'adult' I hate it. Made the mistake of taking to the airport. It was heavy and awkward the whole time.
I might try different styles of messenger bags, or maybe it would work if I had a lighter laptop... But it's a pain to carry any further than a block and a half.

Consider something with wheels. It's really the only way to guard against shoulder strain. I have a number of incredibly ugly, corporate logo'd small wheelie bags that are crazy great for saving the back and getting me through urban settings and work events in a way that a messenger bag can't.
Now if there's a hack for making these pretty, I'd like to see it.

@techgirl wheelies are for weenies.

i loved the look and the feel of messenger bags but now i only use a backpack because you never know how much more stuff you'll need to lug around.

when you have to take more things home then usual - a messenger bag becomes quite uncomfortable.

Messenger bags are much cuter than backpacks, but the backpack option is way way way way way healthier. Be kind to your back, get a decent looking backpack.

I think the benefit of a messenger bag is you can flip them around front and take stuff out of them with two hands while carrying the weight of the bag on your shoulder. You can't do that with a backpack. You have to awkwardly hold the backpack with one hand and rummage with the other.

To avoid shoulder strain I recommend swapping sides occasionally, or only using the messenger bag for light loads and using a backpack for heavier loads.

I like backpacks that fold out into lawn chairs. ;)

A backpack is a pain when you want to get something out of it quickly. A messenger bag is a literal pain in the neck.

I think I've found the best of both worlds with Tom Bihn's Buzz bag: http://www.tombihn.com/page/001/PROD/100/TB0152

It is kind to my muscles like a backpack, but swings around front when I need to retrieve stuff. It doesn't hold an awful lot, but the construction is top notch.

Until this blog posting I did not even think there was an active debate.

Backpacks are for campers and school children. If you are a female a large bag with padded straps should be more then enough.

A messenger bag is good for romping around the city, but a nice leather briefcase or utility bag is ideal.

My camera bag is the awesome messenger style Crumpler Six Million Dollar Home. Great bag, but a little on the heavy side.

But for everyday lugging of books and my laptop I have used the same Dakine skateboard bag for about six years. Best backpack ever.

@JamesM But think of the shoulders, will you? The shoulders!!

i've been using a Manhattan Portage messenger bag for the last year and love it! I only carry my laptop and a leather portfolio to work in it so it fits what i need just fine and no shoulder strain. if, for whatever reason, i need to carry more, there's a zipper that doubles the width of the bag. great idea!

all these people with backpacks. strange. unless i was still in grade school or traveling across europe, i couldn't imagine using a backpack.

cheers,
M

I've been carrying a timbuktu messenger back to and from work every day for the last two years. I keep a laptop, water, and several other small items in it and have never had shoulder pain. Not even shoulder discomfort.