The 20% boost in prices is about right - though obviously it doesn't apply to all categories of stuff. (Wine, for example, is much cheaper in France, tax or no tax.)

The issue of finding a job is the biggie for most Americans, whether or not they speak French at a usable level. We have loads of Brits through here, looking to find a job within the expat community while they learn French. There just isn't that much work available within the expat community - plus when it comes to jobs, even expats fall under the French labor laws (35 hour work week, cotisations, legal holidays, industry labor contracts, etc. etc.).

On the income tax, you file in May after the end of the tax year and you receive your assessment in August or September. Then, you make payments over the next few months - normally in at least 4 or 5 installments. Once you've been assessed once, you pay a sort of estimated tax the following year based on what your tax bill was. Those payments you can make quarterly or monthly. It's a little complicated at first - but falls into a routine once you're here and in the system.
Cheers,
Bev