Hi Julien!
Yes foreign inverstors are most welcome and needed.
Regarding all other questions the best answer I can think of is "it depends". If you are about starting a company that sells to the local tunisian market, I don't know.
For an exportation oriented company, I have some experience to share.
Formerly, a 100% export company had to pay no taxes for the first 10 years (except for salaries and retenu a la source). As far as I know that changed for new companies and is now at 10% which is still attractive.
The process of registration is - in theory - easy and fast. The API offers an all-in-one service where most of the registration can be done. They promise(d) that a company can be started in 2 weeks. I started my company with a tunisian partner and I left the administrative tasks to him, it took us about 3 months to finish the registration of the company (whether this is due to the baroque complexity of administration procedures and corruption or due to my partner I don't know). The paper work we have fills an entire file.
If I would do it again, I would do it myself, however, I would take a good holiday before I'd start the rally. There is companies offering professional registration services. I don't know how professional they are, they charge between 2 and 10K euros. If they would keep their promisses, it would be worth it. If you do it yourself, plan at least a one month stay in Tunisia.
The API has some offices abroad. They don't reply to email (nobody ever does), but if they have one close by, they can provide you with the details.
http://www.tunisianindustry.nat.tn/en/home.asp
Ease of recruitment again depends. There is masses of jobless people, so it should be easy to find employees. In my field (IT) it was and is quite hard to find qualified people, but that is probably an individual problem (Most of IT business in Tunisia is the typical low profile off-shoring kind, so its difficult to find good programmers w/ experience here).
Salaries and other expenses are relatively low in comparison with European standards (maybe 20-50% overall). The productivity is at something like 50% (in case of services such as internet, it's a good idea not to pay in advance where possible and to always have a fallback in place. On the last count, I use 4 internet providers and now I rarely have problems).
If you are planing to move to Tunisia: That's probably a good move. Living here is fun and nice. If you are planing to buy a car, its ridiculously expensive. As investor, you have the right to bring a car and register it without paying the taxes. This however is the most painful process that I ever witnessed. It's probably better not to try it and to just pay double price for a local car. Don't bring your own car before your company is registered, that would set you up for problems.
I made a lot of frustrating experiences, at the same time, this is an awesome country to live in and to do business. Things are settling a bit for me, and now some of the advantages that Tunisia offers are becoming evident. The first years have been a pain in the backside though.
Michael