I've seen it a couple of times on big community websites when people complained about a new design, and then a week later realised that just because it started off not feeling like the site they grew to love, it didn't mean it wasn't a huge improvement.
They key is that, after you force users to switch, it has to be good enough that they realise you were acting in their best interests - if you force them to switch to something that really is much worse, that's when it becomes a problem.
I was wondering: if someone used the Twitter API to create basically a clone of the original site, would that be against the Ts & Cs, and if not could this be got around somehow. For example if someone made it and open sourced it, I could host it myself with my own API key, and without marketing it anyone other than myself and my friends Twitter wouldn't know or care?