Hey Stafsel,
Are you sure you would consider this lens ?
This lens is trying to combine the 18mm (extreme wide angle) with 400mm (extreme tele). This is asking for problems like vignetting, soft focus and chromatic abberation. The common rule of thumb is that the zoom factor of a decent lens should be 3x maximum. This means that for a 70mm minimum, the maximum should only be 210mm. That is why 70-200mm and 24-70mm are such popular lenses. If you exceed this rule of thumb, the image quality is likely to drop. The new Tamron 18-400 exceeds this advised zoomfactor with a dazzling 22x zoom.
Check out
the Digital Picture website too see the comparison between the existing Tamron 18-270mm and a standard Canon 300mm/f4.0. All you need to to to compare the photographic result is to move your mouse on and off the testshot.
Personlly I expect the results of the new 18-400mm to be worse than the above test. I don't know if you wanted to hear about this opinion, but at least you can compare some lenses now.
Cheers,
Iwan