Is the Nikon TC 20E-III , I assume your talking about the picture quality,Yes i'm happy with it but i've only been using it on the 70-200mm F4 which is a pretty decent lens,same as most things there is tradeoff's involved,Losing 2 stops does slow down the AF when there's not great light + you do lose some detail but overall i definitely don't regret buying it.
The versatility of being able to get to 400mm without carrying around a rocket launcher or the cost of one suits me fine for now.
If one was a professional action shooter i think a F2.8 lens would be a wiser choice than the F4 but for me its ok.
Did post a couple of shots with & without the 2.0xTC on the 70-200mm F4 here
http://mybroadband.co.za/vb/showthr...hotography?p=12010853&viewfull=1#post12010853