Transformation Card - Why do people claim it is so good?

T-card pairings depend entirely on the power set and loadout. If you have a power like Electricity where you can do the long range stuff without having to do a lot of movement, you will probably do something like T-Card/Eye/Scrap so you can rebuild your supercharge and pop it (Circuit Breaker) more frequently, thus increasing your damage output. The same would be true for Rage and Earth might-based melee loadouts.

For most powers with a range loadout, yeah, T-card/Strat card... and possibly something like Solar Amplifier (for might-based), Grimorium Verum (might or precision when not in a strict single target), or possibly the LaMort Card (strict single target).

Yup. I find that it makes a great paring on my Munitions and Gadget toons