Tekkit was very fun for a while... Was crazy seeing the amazing assembly lines converting raw mats into other mats and placing them automatically within boxes. Problem with Tekkit for me, is that it completely diminished the essence of what Minecraft was about. To me, on vanilla mode at least, MC has always been about exploration, and building. The instant we start a new server, every time on vanilla at least, everyone looks for the coolest caves / mountains to build their shit in. The simplicity of vanilla is that as soon as you log on, you can keep building and exploring.
With Tekkit, while it was fun at first, it seems like over time you're spending more and more time glued to the areas where you have built your machines, and you're spending tons of time just to tinker with those machines. As someone who played casually for an hour or so, I was getting simply depressed at the thought of logging back in to try to play, because that meant always going through a time consuming routine of breaking out all my machines / tools to start mining. By the tme I felt I was set up, I'd have 30 minutes left to play. Yea, I could just go out there old school with a pick and axe, but when you saw people quarrying out huge chunks of land, what's the point? Tekkit certainly is interesting, but again, to me it's not what Minecraft is about. I consider Tekkit to be more about building shitloads of pipes and machines, whereas vanilla MC is about exploring and shaping parts of worlds.