[inquisitor]

Originally Posted by vegemite

What do you mean emails use up credit faster than regular browsing?

Those "Internet Max" options include only traffic over the ports used by http for transferring webpages (80, 8080). All other connections will be billed separately at a prohibitively expensive rate of € 15/MB (billed in 10KB-increments, "Pour les autres usages dont les usages mail (smtp, pop, imap) et GPS").
So you can browse the web without limitation, but as soon as you use an eMail client or Google Maps or any other application, that uses other ports than those mentioned above, your credit will disappear quickly.
If your eMail provider has a webmail interface, through which you can open your eMails in the browser instead of using an eMail client, this will be included in the "Internet Max" options, since technically webmail is just a web page.
You can also surf the web using a computer with a 3G modem under those "Internet Max" options - when I tried that back in May it worked flawlessly and no additional charges applied. However be sure there's no other application running, that would establish IP-connections on non-web-ports (e.g. Skype, antivirus updater etc.; Windows Update is not critical, as it runs over port 80). Also there's no need to manipulate the USER_AGENT of your browser.

A friend suggested a store called 'Leclerc'? Is this right?

Leclerc is a chain of super markets, which also have a MVNO-product called "Leclerc mobile". According to their price list they charge € 1/MB (billied in 1KB-increments).
Btw the French abbreviate kilobytes with KO, megabytes with MO and gigabytes with GO.

I still have NO idea what SIM to get in France, they all seem hideously expensive and hard to get, or am I wrong?

You are absolutely right. That's why France - among other countries - is considered being part of the "axis of wirless evil" on this board.
For A-GPS, which generates just a few dozen kilobytes of traffic each time you start a GPS-app, Leclerc seems to be the cheapest solution.