Guess it won't hurt to repeat this in this thread for reference. Here's an illustrated difference between < $100 and > $700 300mm lenses with a 1.4X TC.
The Tamron
AF 75-300/4-5.6 LD (672D) at 300mm and f 8 with the Tamron 1.4X PZ-AF MC4. Cropped to 100% and lightly sharpened in Faststone Viewer. That's an effective aperture of f 11, one stop above actual max aperture. There's a very noticeable softening at f 5.6.
The Tamron
Adaptall-2 SP 300/2.8 (360B) at f 4 with the Adaptall 140F TC.
Same PP-ing. Effective aperture f 5.6 here. The image sharpness improves between f 2.8 and f 3.5 actual.
Both off a sandbag rest with the SSLTT (Super Sophisticated Lens Test Target) at about 100 yds. APS-C FL is 600+mm for both.
Here's the normal view through the SP 300 without the TC and before cropping. The holes in the grill are about 2mm in diameter.
With hand held or makeshift support in the field, and under practical circumstances and good light, the economy 70-300mm zooms can produce credible results compared to lenses that cost a LOT more. They're most certainly good training wheels while learning the quirks and special tricks (mentioned above by Blue) that frustrate new tele lens users.
H2