Amber,
First, it all depends on the context. I am all for a focused training during which you do little else. As for the location it is not important, as long as it is legal, weather permits and within your budget.
I did mine in Florida (at EFT) in 22 days. That is 22 days of acceptable weather for PPL training. Florida is not sun and fun all year round.
Read indeed the sticky above from Backpacker. Now, in reply to your questions:
1/ in 2010 I was very pleased w/ European Flight Training in Florida.
2/ 3 weeks: see above. I would say take 28 days min. Do not underestimate jet-lag, weather, ... .
3/ back then it was about 20% cheaper for me.
4/ definitely worth the money and time for me. Won't forget this one, even if you do not fly much after that. But in my case I would not have had to opportunity to co-ferry a plane from Belgium to Thailand had I not done it
Good luck, just go for it.