Hi
After having spent six months in short term rentals in Paris and having looked through several agencies for a longer term fully furnished accomodation , here is some feedback.
First from the agency I rented from those 6 months, about which there is very little information on the net :
http://www.apartment-living.com/
It's a small agency with only few apartments, especially larger ones, but things went smoothly, before the rental, during and after. Note that their website is on the small side as well, they multiply by 10 from sqm to sqft hence you can add 10% to the surface advertised.
The biggest parisian agency seems to be
http://www.parisattitude.com/ but many of the places they advertise are not available and I just got to visit a couple of apartments despite asking information for many more. Yet they look quite serious.
The best website would be
http://www.book-a-flat.com/ . Besides the 2 apartments I visited were very similar to what they advertised. Somehow the agent didn't manage to find an agreement between the owner and myself in time and I ended up renting elsewhere but i would recommend their services - and the following apartment I wanted to rent is still available btw, a very decent deal by parisian furnished apartments standards
http://www.book-a-flat.com/en/apartment-paris-2088.html.
Another important agency is
http://www.lodgis.com/fr/ , they have probably the cheapest agency fees and don't even come to the visit, but rather let you talk with the owner, which might be good to help sorting out details quickly (in contrary of book-a-flat, where there was always the agent in the middle and we wasted too much time to discuss minor details). Yet this agency's methods are bound to create problems. Not only they keep on advertising apartments long gone which is a little sin shared by several other agencies, but their notation of the apartment's quality is a bit suspicious. Visited through them a 150sqm apartment which was by far the worst I'd seen in that period in matter of equipment. Looked hardly better than a campsite inside with dirt cheap furnishing whereas it had their best 4stars rating. Told that to the agent that said they would look into that matter yet months later it was still advertised on a couple of websites with the same ratings. Much better to visit the apartment before paying anything under those conditions. Besides the agent kept emailing me, offering again and again the same apartments in different order, which was upsetting.
There are many more agencies which I have less experience with but hope the above will be of help to other forum members.