The Big Green Egg is a good choice if you are just getting into slow cooking or have a limited budget. They do a great job and are readily available. The drawback to the BGE is that is does not operate at as low a temperature as some of the others (Kamado and Komodo). The Primo appears to be similar to the BGE though I have no expereince with them.
I "upgraded" to a Kamodo cooker and have had great success with it, even running it as low as 180 degrees for smoking salmon. Hard to get a BGE to run that low. However, I caution you to NOT buy a Kamodo cooker as the company does not operate faithfully any longer. The cookers are lower quality (now made in Indonesia) and the company will take your money and not send you yourcooker for many months, if at all. And do NOT order spare parts - I ordered $600 of spare parts in January of 2008 and still do not have them (July 2009). Emails and phone calls go unanswered. Many others have the same expereince - check out www.kamadofraudforum.org for the horror stories.
The Komodo Kamado looks like a superior (but expensive) product. As a avid Kamado cooker user, being intimately familiar with the construction flaws in their cooker, it appears that the Komodo product fixes each and every one of them. After about 7 years of use, my Kamado is failing (leaking smoke THROUGH the ceramic tiles!) I plan on replacing it with a Komodo brand.