At least for Shimano, most folks find 105 and up (ultegra, DA) to be very similar - almost identical in performance. The good stuff weighs a hair less but costs a ton more.
The lower end stuff like Sora or Shimano 2200-2300 is definitely not as good as 105, but you'd be wrong to say it's bad. In fact, it's a really solid performer. I have a Sora bike that I train on less now but used to train on a lot, and it's required no maintenance and still works great. It has never limited me.
I still find it interesting that people find it so effortless to drop $700 to upgrade the shifters because 'it's smoother' when in reality, the Sora / 2200-2300 stuff is actually far better than most riders will ever need. (Criterium racers should skip the Sora stuff and go straight to 105+ though. Tons of hard shifts in those.)