A couple of us have tried the Rainbow brand "Homebrew" of Root Beer flavor and both of us agreed the aftertaste had a highly "medicine" flavor that was not pleasurable. I found the Zatarains didn't have that harsh aftertaste. you can get the Zatarains from either the company itself (very good price) on it's web site or a few other places online. I tried the RIO rootbeer flavor from Prairie Moon
http://prairiemoon.biz/fulflavlissa.html it seems a little weak for my taste. The nice thing about this place is they have MANY different flavors and the prices are decent, I got an assortment of 12 flavors to try out. I haven't had a chance to pick up the McCormicks extract from Walmart to give that a try. The other brand I have heard decent things about is Gnome brand which is available from home brewing shops and several places online.
I have moved from using yeast carbonation to using a carbonator cap with a C02 tank and 2 liter bottles. I wasn't happy with the yeast flavor in the finished product. Also I don't have to wait 2-3 days since this can be carbonated quickly )within minutes if you have already cooled off the mix. Soda's carbonate best when soda is cold.
I was able to get a used C02 tank and regulator on Craigslist for under $100 and then a 20 lb Co2 fill was $18.50 at the local welding gas supplier. New tank/regulator and fill would be around $190. In some cases on Craigslist you might find someone selling a cylinder with Co2 still in it. At the welding place they just trade out the empty tank you have and give you a full one. I highly encourage you get a Aluminum tank of the 20 lb size or less so they are more easily handled. From what I have read I should be able to carbonate between 600 and 700 2 liter bottles from one 20 lb Co2 tank. You can save a little money by going for a 5 or 10 lb Co2 tanks size, but when you get them filled they all cost similarly for a refill so you save money going with a 20lb tank from that standpoint. Yes, going this route is not cheap BUT you can also move on up to making up 5 gallon batches with Corny keg (available at most homebrew shops or someplace like
http://www.cornykeg.com/) for only another $30 or $40 investment in a tank and an inexpensive outlet tap. The downside of using a corny keg is if you make smaller batches, less than 3 gallons, you end up wasting a lot of Co2.
So I am trying to perfect my favorite root beer in 2 liter bottles and then once I get to a recipe I like then I can move to a keg system and make 5 gallons at a time.