And how much?
They seem to have and agreement with Orange for national roaming on 2G and 3G. They have there own mobile service. But according to Wikipedia the coverage is patchy. Have you checked the coverage in your area?
On (short) holidays in France I usually roam on Orange or SFR and was very satisfied. Have you checked their service?
I use Orange also. I get a SIM for the iphone and iPad. Very good coverage. Going again next month. I always stop by the Orange store on Champs Elysee in the first block from the Arc de Triomphe. They have English speaking staff. Very helpful. Use Google translate for the web pages in French.
Their phone service is like ours. Cheapest is not always best way to go. Having lots of data does you no good if the coverage is not where you are.
Over 15% of french SIMs are on the Free Mobile network, their coverage has gotten better in 4G and 3G, and anyway you still get transparent roaming through Orange's network in non-covered areas. It all works quite very good.
For 20€ you indeed get 20GB of 4G datas at full speed, unlimited sms/mms in France, unlimited calls with France/USA/CDN/China and unlimited calls on landlines with 100 others countries. They also offer free roaming with same services in whole EU + few other countries (CDN, Israel, ..) limited to 35d/year in case you plan to move out of France frontiers (but then only 3GB in 3G, and unlimited local calls/sms added).
But I don't know and couldn't find clear infos if non-resident can subscribe. They have got stores too where to get more infos once you're there/here, or you can try to call them of course.
Europe was more advanced than the US in 3G coverage, I think; 3G is fine in France in general as long as you're in a city at least (and roaming between the different networks when necessary). 4G coverage is still building but tends to start to be ok in most crowded areas/cities.
Free shaked the french 3 operators mobile landscape starting from scratch in 2012 (it first was only an ISP also kinda avant-gardist regarding tarification and services in its time), built —and still is; their own coverage map has a bit of ancients datas— its network of mobile antennas and a strong and still growing user base with attractive prices and services. Over 10M ppl are happy users and probably not looking back.
Free doesn't offer pre-paid contracts. They only have no-engagement post-paid contracts (or should this be called 30 days contract?). Customers simply subscribe for the service and get no subsidies for phones, pay their fee every month, and can leave whenever they want at no cost.
In your case I suppose it should be possible going at a store to get out with a working SIM and a french number, but they might require a physical adress for subscribers (I find only infos about e-mail address and payment can be made with check, but then I wonder how they could get paid any consumption out of contract, like calls on 08XX non-free phone numbers for example). I don't know. And then you'd have to "check out" at the store before leaving France too I guess. If even possible, it could be a bit of a PITA compared to pre-paid concurrents' offers maybe.
Going at a Free store they also might be able to inform you about some pre-paid MVNOs plans for travelers in your case, but you probably won't get a match with their own service.