(Sorry. I just couldn't resist the pun.)
Everyone's going to need a way to carry their kid on their body, unless they're adopting a kid over the age of 3. It's my opinion that you really need two carriers--one ring sling and another carrier. Let's discuss the ring sling first.
A ring sling (I'm also including pouch slings in this category) is a simple design that has a bunch of advantages as a carrier (and a bunch of flaws, but that's why you'll want another carrier, too). It's relatively simple to use, you can use it to carry your baby in several different positions, it's not bulky so you can shove it under the stroller or in your bag, you can use it as an emergency diaper pad or pillow, in some positions you can nurse in it, and (most importantly if you have a toddler) it's super-easy to get the baby in and out of it (and in and out and in and out...).
Some people like a lightly padded sling, but I think an unpadded sling is more comfortable. Padding is supposed to make it more comfortable on your shoulder, but the padding IME makes it bunch so the weight doesn't really distribute itself evenly. Lack of padding also makes it cooler in the summer, and far easier to adjust when it's on.
When I had El Chico, the only unpadded ring sling out there was the Maya Wrap. (Now there are a million people selling unpadded ring slings.I haven't tried any of them, but they're ring slings--you can't go wrong.) I live in New York CIty, yo, so I couldn't walk around with stripey granola fabric, so I was temporarily stymied until I found the awesome and oh-so-simple-in-both-design-and-execution pattern to sew my own sling *. I far prefer the shoulder design of this sling to any of the ones my friends paid money for. And it takes literally one hour (including pinning) to make it. Order the rings from slingrings.com (craft rings from a craft store are not strong enough and your baby could get seriously hurt in a sling made with craft rings). I recommend nylon, because metal are heavy, can bang you in the jawbone or your baby in the sweet little soft head, and make a nasty noise in the dryer.
A sling ring will run you anywhere from about $10 (if you sew your own) to $120 (for a silk one). Most are in the $25-40 range.
Do not under any circumstances be fooled into buying the NoJo sling that they sell on Amazon.com. It is beyond dreadful.
The Other Two Carriers
The real problem with a ring sling is that it's not going to be comfortable for long walks. For around the house or quick walks it's wonderful, but if you regularly carry your baby (or toddler) around on your body, you'll need something that distributes the weight better onto your hips and back and off your shoulders. There are two carriers I've never heard a bad word about: the Ellaroo wrap and the Ergo carrier.
The Ellaroo is the one that I own (in black, of course). It's a long piece of woven fabric that you tie in a bunch of different carries, so it works for a newborn up to a 3-year-old (on your back). It was so perfect and secure when El P was a newborn (and I could walk around and nurse him and no one had any idea). Now he hangs in it like he would in a Björn (facing out or in depending on mood), except it's actually comfortable on my shoulders and back. When he's older I'll carry him on my hip or back in it. The woven fabric gives it more strength than a stretchy wrap like MobyWrap, Hug-a-Bub, or Ultimate Baby Carrier, so you can use it up to a higher weight and older child. It's cool in the summer (even the black one), and a bargain at $70. If you buy your from a local distributor instead of off the web, she'll show you how to tie it and put your baby in it for no extra cost. The disadvantage of the Ellaroo is the learning curve when you learn a new way to tie it.
I have some friends who love love the Ergo Baby Carrier. It puts the baby's weight right on your baby's hips, so no more aching shoulders. You can use it to carry the baby on your front, hip, or back, and it's easy to put on and put the baby in. This is a great choice for people who like the feel of a backpack carrier, but don't want the extra weight or metal frame of an actual backpack, and for people who are hesitant about tying a wrap. It's a bargain at $92, and you can buy it on the web or at a store near you. The disadvantage of the Ergo is that it's pretty bulky and weighs more than an Ellaroo, and isn't as easy to nurse discreetly in.
Carriers I Don't Know Much About
The MeiTai is a Chinese-style baby carrier made by Ellaroo, Colibri, or a bunch of other companies or WAHMs (search The Babywearer and/or Ebay to look for different models). The MeiTai puts the weight of the baby across your chest (if you're wearing the baby on your back) or on your back (if you're wearing the baby on your chest). I've heard some people say they loved theirs, but don't know much about the features.
The Sutemi looks a lot like the Ergo to me. I ran into a mom at the airport a few weeks ago who loved hers, but, again, I don't know the specifics, except that you can't use it before 4 months, but they say you can use it up to 5 years(!).
Carriers I Don't Recommend
The Björn. $120 for something that you can only use for one carry style, can't nurse in, and that will cut off the circulation to your arms once your baby is 18 pounds? Next. (They make an awesome potty, though.)
The NoJo Ring Sling. It stinks like fabric dye, and is impossible to adjust while the baby's in it. Too much padding, and hardly any room for the baby. Resale value on Ebay = $0, so that ought to tell you something.
The P-Sling. $550 for a "hand-crumpled" linen sling? Wait until the first time your baby spits up on it. Although maybe the children of people who can afford $550 for a sling don't ever spit up.
The Hip Hammock. Absolutely kills your neck, and it doesn't do anything you can't do better with a ring sling.
For more than you'd ever need to know about slings, register at The Baby Wearer and check out their bazillion reviews. Have fun wearing your baby.
*Can you believe people are taking Jan's pattern and sewing slings from it for sale? It's called intellectual property and common decency, people.