Got back from my short trip to Paris and wanted to update on my SIM card experience. I already had my Lebara SIM number and topped up online with £10 - used this in an iPhone 4 as backup (and for calls back to the US since that was only £0.05/min). When I landed in Paris, I made a call with the LeFrenchMobile SIM card in my iPhone 5, received an instant text message with my new number, texted back to add the "Special Deal" 50Mb/week data package and was up and running right away.
I used the two HolidayPhone.com data-only SIM's (they used SFR) in my Mifi device for main data access between two iPhone 5's, and everything worked well. Had decent 3G coverage in most of central Paris though it dropped to "2G/E" in parts.
So it all worked great, and it was nice to be able to make/receive calls and use data as needed.