First and foremost I have to say those are some very lovely shots...very well done indeed! I too am a Tamron and Sigma fan although I'm a Canonite instead of a Nikonian
. While I am looking to get a Canon Image Stablized L lens or two, I've been quite happy with my currant lenses and especially my Tamron 70-300mm which has been my main lens. I'm actually considering their new 28-300mm "VC", but I'm waiting to see what these compatibility issues are with Canon and if they get them ironed out.
The -ONLY- negative comment that I have here is that on the first shot you have a "spot" just to the left of the flower stem...looks like either something on the lens or possibly something in the background. If it were me, I'd take a moment and clone it out in your software. Beyond that, you have some great compositions there, wonderful color and sharpness, great DOF...very nice.
I would also like to add that in regards to your comment about some not turning out and some that did...don't sweat it! Many of us learn and shoot by trail and error and there are even "pros" out there who are happy when they get 1 or 2 really great shots out of every 20 or 30! It's the great shots like those you posted here that count!
Anyways, welcome to the forum...I look very forward to seeing more!