Is Flash a good rapid prototyping tool for UX?
A lot of effort small reward. You can port vectors from fireworks to flash if there's a chance you'll end up in flash world or not but I think fireworks is better than flash if rich prototyping is what you're after http://www.guuui.com/posting.php?id=2112
If you're following an agile methodology and looking for a quick hit, why not consider some of the products that allow some level of prototyping to be built along with wireframing your application/site?
On Mac, Omnigraffle has some crude clickthrough capabilities in presentation mode, even allowing the loading of canvas layers to mimic states for the same page. Or you can build very solid rapid prototypes using Axure, which comes in Win and Mac variants.
There are also several online applications that do this to one degree or other. Take a look at Balsamiq, iPlotz as starters. Still others like Creately work pretty well too.
As for Flash, why on earth would you even consider that when you are looking for 'rapid' ... won't even go into the usability issues.
If you are looking for something to create highly interactive prototypes with you might also want to check out LiveCode by RunRev (http://www.runrev.com/products/livecode/). It supports delivery on a wide-range of platforms, including Mac, Windows, Web, iOS, and a very early beta for Android.
The programming language is fairly user friendly, and the storyboard (card) metaphor works very well for the purpose of building scenario-based prototypes.
This is like asking if a piano is good for a musician to write songs with. The answer is, "It depends if they know how to play piano." If they are a guitarist, the answer is no. If they are writing a song for the harp, the answer is no. If they need to do it while flying on an airplane, the answer is no. If they don't own a piano, the answer is no.
Flash is a tool. If you are proficient in it, and the output is Flash, and you have a copy of it on your computer, and you think it's a good choice, then sure, why not.
If you're just looking to create wireframe mockups for your UI, I would recommend one of Balsamiq's tools.
If developing a Flash/RIA site, Flash Builder isn't too hard to work with in my opinion. I wouldn't use it to mock up a regular HTML web site though.
For websites in general I would definitely NOT recommend it in any case, though – native is most often best. For mobile and tablet apps, Webkit kicks quite some ass, and for anything that requires more advanced capabilites (e.g. exhibition installations), I would highly prefer to use e.g. Processing or openFrameworks.
Wow. Quite a lot of anti flash-sentiment here. The answer is that it depends on your skill resource, if they are any good then they can present behaviour in flash to mimic any environment very quickly.
Axure and the like are very limited to present hi fi solutions for user testing for any projects aside from next page functionality.
SO the answer is theortically its a brilliant option if you have access to the skillset, if you dont, then dont.