For me, the writing was on the wall when Twitter for iPhone first came out; they took the relatively simple and easy-to-use search screen and combined a bunch of separate functions into one bizarre combo search box that felt unintuitive and annoying. I still used it for a while, but wound up switching to the delightfully intuitive (if a bit under-featured) Twitteriffic 3.
I kept hoping they'd turn it around, but for me, Twitter for Mac was the death-knell for Atebit's aesthetic; what used to be an elegant and beautiful, yet still right-at-home on the Mac experience turned into an ugly, user-hostile piece of software… just like Twitter for iPhone.
RIP, Tweetie. You will be missed.