I love, love, love my Mei Tai! I honestly can't say enough good things about it. I only ever used the back carry once my kids were past about 9-10 months old, but we still used it quite a bit. I bought mine from a work-at-home mom when my middle child was about 8 months old, and it has gotten so much use.
I carried my middle daughter in it several times per week until she was 18 months or so, and after that, I carried her from time to time until she was 3. I carried my oldest in it occasionally up until age 3 1/2 as well. It was perfect for trips to the zoo or long walks where they wanted to walk some and be carried some.
My son is now 18 months old, and he rode in the Mei Tai at least an hour a day as a newborn up until about 6 months old and then still a few hours a week until a year. He'd ride on my back at the mall, grocery store, on walks, etc. It was great b/c he could just put his head down and go to sleep whenever he was tired. He was an early walker, though (10 months), and hasn't wanted to ride in the sling lately b/c he wants to be like his sisters. I still think we'll use it on zoo and amusement park trips this summer, though.
Anyway, I think a Mei Tai is still a great investment for you, especially since your son isn't walking yet. And if you have any more kids, you'll be able to use it again.
Oh, and if you or anyone in your family can sew, it's not really that hard to make a pouch sling or a Mei Tai. Just google some instructions, and it can be cheaper than buying one. Just a thought...