Updated January 2006
Some U.S. national parks/forests (the High Sierras in particular) now require the use of park-approved bear-proof containers in problem areas. Be sure to verify with rangers before you pack! In problem bear country you're generally limited to using either a site's bear box (when available) or lugging along a bear can (see suppliers below). In the Sierras, counter-balanced food bags are now considered "food pinatas" for bears - it provides entertainment before they inevitably pounce and feast on the contents like over-excited kids at a birthday party. 600lb furry kids with sharp teeth. On my John Muir Trail thru-hike I gave food on the second day to two backpackers who had a bear push over the tree they'd hung their food in. Cubs are now taught by momma to crawl up the tree and "dive bomb" counterbalanced food bags, so even the strongest tree won't help.
For those heading into the Sierras, you'll want to visit the SIBBG website (Sierra Interagency Black Bear Group) for one-stop shopping on all current bear-related backcountry information. Includes links to list of approved containers for the various national parks, packing tips, etc. Remember, if you can get from bear box to bear box, you may be able to avoid carrying the can (or at least get away with carrying the smallest lightest one you can find). Just be prepared from some potential flack depending on who is sitting behind the desk when you get your permit.
When maps aren't provided, be sure to call the Ranger station to verify the described areas. I just had a pleasant conversation with a staffer at Sequoia Nat'l Park where we both worked through the topo trying to fully define the bear can zone in the SE of the park. It can get a little fuzzy, when in doubt ask!
Some sites offer bear-proof lockers ("bear boxes"). These brown metal footlockers are my best friend in bear country. If you can limit your itinerary to overnighting in those designated areas, you can usually avoid carrying a can in the control zones. As a starting point, Climber.org maintains a list of Sierra bear boxes. Sequoia and Kings Canyon Nat'l Parks have a list of bear box locations as well.
Things to remember about bear boxes is that you're only allowed to store food and scented items (toothpaste, etc.) in them. They are NOT for hiding your pack while you dayhike. Also they can get pretty full depending on location and season. I've never had a problem ultimately getting my food to fit, but just keep that in mind.
Finally, the bear box is only as good as the people in the site. If someone forgets to close the box up, even for just a few minutes, prepare for rodents, moochie deer and fearless bears to come out of the shadows like ninjas and run off with random booty. That could be a serious drag if it was your bag, through no fault of your own.
Many parks offer various bear cans for rent if you don't want to purchase one. Bear in mind the cans take up a LOT of space, and that ends up being dead space as you eat through your food. Some will claim that they use the empty space by storing other items (even clothes - yikes) in the can. Sorry, but the last thing I want any of my gear smelling like is food. Your decision, that's just my opinion (and why I like the Ursack).
If renting, try to fit a same-sized object into your pack before you leave, otherwise you may be in for an unpleasant surprise at the ranger station when you pick up your rental can. Carriers exist for keeping them outside your pack, some less awkward than others.
I used the Garcia many years ago. I cursed its weight and awkward shape (a slightly bulging barrel) from Day One. If you're persistent you can fit one into a GoLite Breeze, but it's not pretty. In the summer of 2002 I used my first Ursack in the High Sierras, and I was very pleased. Chris, Jas and I shared two TKOs and one "Classic" Ursack, we never had a problem with bears or rodents. The TKOs were easier to properly tie off, and watching the Ursacks compress down as we went through our food supply was a joy.
Personally I'm a huge fan of the Ursack TKO, to the point that I'll hike longer days to get through "mandatory bear can" country in order to avoid having to lug a can along. Now that the TKO 2.0 Hybrid is an option I'm looking forward to trying it out too. I have received email from a reader whose older Ursack was gnawed through by rodents. I tie my Ursack well above the ground and exposed on a solid section of tree trunk. I think that's a fairly uncomfortable position for a rodent to spend much time in (hungry owls and such). I don't know if it matters, but I haven't had any rodent problems either. Anecdotal evidence is often weak evidence, but that's my experience to date.
Outside Magazine also has an article on the Ursack that may be worth reading.
The pros I see in the Ursack are that it weighs much less than any can for the same volume, takes up only about as much space in your pack as your food (which means less space consumed as your food diminishes), and is quite cheap. The cons are that the bear might still gnash and gnaw on the bag, rendering the contents mashed, ruptured, and gooey, and the bag-only version's lack of approval in the High Sierra's nat'l parks. I'll see how the TKO Hybrid 2.0 fairs on my travels in SIGGB Land and report back. The true "cans" are the bomb-proof solution at the cost of increased weight and a fixed volume (which becomes dead space as you eat through your food).