i love slings!!! there are a ton out there and they all have thier merits...
Ring Slings are great for quick trips into the store etc. They are esay on and off, adjust quickly and easily (as long as it's not padded) and they are usually fairly easy to nurse in. They do not do so well for really long carries (like walking around Disney land) as one shoulder is bearing the wieght.
Mei Tai is a little harder to on and off than a ring sling, but no more difficult than any conventional carrier (like a snuggly or baby bjorn) my newborns hate them but they are great for older kids and it's easy to carry a toddler in these. You only have two ties to worry about and nursing is usually pretty easy these as well
Wrap these are my all time favorite. They are the most versitile and if you can hem a piece of fabric you can easily make your own. There is more of a learning curve as you figure out the ties, depending on the carry you use you might have to completely unwrap to nurse. They are the best for long carries as you can distrbute the weight between shoulders and lower back. They come in a variety of fabric types depending on weather etc. but like I said you can make your own if you can hem, you don't even have to hem if you use a knit fabric. I really like the tying instructions at gypsymama http://www.gypsymama.com/colorbooklet.pdf