I am going totally against the grain here from the looks of these posts. But I would recommend mixing your own soil from scratch. I say this because I live 40 miles from the nearest grow shop which are way over priced to begin with. All this talk about this number mix and that number mix makes my head spin. Look at what most of those products primary componiets are a few different things and in almost all case incomplete to begin with meaning you will have to ammend everyone of them to grow anything in it. So why not buy all the items in bulk and seprately to begin with and in the end you will know exactly whats in there. There are no regulations on those companies that sell those premixes either meaning they just throw whatever is laying around in there. Go to a local gardening center save yourself alot of money.
There's definitely nothing wrong with mixing your own soil, that's a very good point. My only caution on this is that it takes a little bit of work and research to make a good mix. Couple of recommendations off the top of my head:
1. Keep biology in the foremost of your mind when making your mix. This means 25% of your mix is a high quality compost or earthworm castings.
2. 25% should be for aeration, meaning perlite, pumice, calcined de, pyrolized rice hulls, etc....
3. Peat is a good option for the other 50%. I like Alaska or Premier brand.
4. Don't forget to add nutrients and minerals. Many people don't get good mineral content in their soils. There's much better options than lime.
5. Let your soil cook before planting.
6. Less is more in most instances.
7. Kelp meal is a must-have in my opinion.
Just what I came up with while typing my response. I'm sure there's more I'll think to add later, and this is a really rough guideline....