I love TE for the customization, the various interesting machines that are amazing for automated setups (getting water, snowballs and cobblestone can be very useful), basically most things. Also, the textures look fabulous, and that is something that I care about very much.
Ofcourse, the low impact on your computer is amazing too. Makes the mod available for so much more people.
The only thing I don't like is the easy progression. Getting Invar can be done with only a Pulverizer. The Redstone tier requires Electrum and Hardened Glass, both of which are very easy to get. The only tier that is hard to get is Resonant, but that is because Shiny Ingots are hard to get in an unreliable way. You need to either grind up Ferrous with a rare chance to get Shiny Dust, or use Cinnabar which can only be gotten from Redstone Ore (or TC4 Cinnabar), which, unless used with other mods, is unrenewable. It's fully dependant on luck, which isn't a good thing either.
I'd also like more config options to make stuff harder to craft. Steel is good, but what if I don't use a mod with Steel?
The progression system could use some work, but overall it's an amazing mod. Convenient to use, easy to configure, I love it.