Designers generally shouldn't be worrying about the things Bootstrap covers, like how a button looks. That's a commodity. It always has been. Instead, figure out where to put the button, and whether it should even exist. That is as valuable now as ever.
Furthermore, all the designers I've ever worked with like to throw a PSD over the wall and say, "Make that." Every little detail reimagined and drawn from scratch. They don't know how to build and work from a set of reusable CSS components like Bootstrap. So as a developer, I'm inclined to say forget your PSDs, learn CSS, we're on the web and it's part of the job.
I don't know why how a button looks should be a commodity. However, how a button looks by default clearly is.
In general, for example, I might want a certain set of buttons to look different from another set of buttons because this provides useful information to the user of the system.
I haven't worked with bootstrap yet, however it looks very promising. The key to a good framework is how much it makes what you want to do easy when the tasks align, and how it stays out of your way when the tasks do not.
I actually look forward to playing with Bootstrap to see what it's capable of doing. It looks like the alignment handling might be worth it by itself. However, the article here made me somewhat less enthusiastic. One of our goals is to produce something easily themable. If bootstrap helps, great. If it gets in the way, that would be a bad thing.
Furthermore, all the designers I've ever worked with like to throw a PSD over the wall and say, "Make that." Every little detail reimagined and drawn from scratch. They don't know how to build and work from a set of reusable CSS components like Bootstrap. So as a developer, I'm inclined to say forget your PSDs, learn CSS, we're on the web and it's part of the job.