OK - I'm one of those who has paid a bit for a Tamron TC 1.4 - from one on this thread...
Just waiting for it to arrive, then I'll try it on the back of some of my lenses.
One that I'm interested in trying it on is my Tamron 18-250 which I use as a walkabout lens when I only want to take one lens. Having the TC with me will just extend the range enough.
Went whale-watching last week, with the K20D and the Tammy 18-250 only, and got some nice shots. Those humpbacks got up REAL close to the stern of the boat, but if I'd had the TC with me, I could have got better shots of the whales when they were just that bit further away frolicking.
I shot in RAW, and on auto-drive, popping off 3 - 5 shots at a time, in shutter priority mode, all at about 1/320.
You can see from the third shot that just a bit of extra reach - say about 1.4x - would have given me a nicer shot without having to crop.
All shots taken DNG, saved as JPG in Picasa, uncropped, then scaled down to 1280x1024 in Irfanview