I'm currently building an outdoor oven, and I'm really really tired and sore, so my clarity might be skewed here.
An outdoor oven relies on thermal mass. Lots of it if you plan on doing roasts, turkey, bread, etc. An oven will fire in about 75 minutes, and you can cook in it for many hours. I've relied on 3 websites for my info: Traditional Oven, Ozark Dreams Brick Oven, and to a lesser extent Forno Bravo. For 20 bucks Rado Hand (Traditional Oven) will send you a DVD with hundreds of building photos and lots of information.
You have to like to cook a lot to justify an oven. If you just want it for pizzas, I don't think it's worth your time. And if it doesn't work right, it's cause you skimped on the insulation,,,top, bottom, AND sides!
As far as being a fad,,hogwash. These ovens have been around for many decades and are still holding up to constant use. I have so much concrete and steel in mine I'd almost love to come back some day with a 30 pack and watch someone try and demolish it!!
With a few modifications, this is what I'm building,
The more you learn about ovens, you can tell by this pic that it is lacking in thermal mass. I would suggest lots of reading and studying before you break ground.
I've taken lots of pics, so if ur building one and get stumped, PM me and I can send a pic of how I blundered through a specific problem.
Oh yean,,,and if ur old like me, make sure you've stocked up on Aleve and Ben Gay
Brian