I spent 5 weeks in the Ariege last July/August with family and some friends, starting to build a holiday house. For the duration we all camped outside, I used a hammock for the whole 5 weeks without any problems. I didn't see a massive amount of the area, due to being hard at work most days, but I did manage a few walks and some cycling (admittedly road..

) Anyway, here are a few of my observations, I hope they can be of some help..

I just want to make it clear I can't really comment on the higher areas further south (above 1200m) as I didn't spend much time at all there, only the lower wooded areas.
Weather - Sleeping at an elevation of about 1000m, near to Massat, in a heavily wooded area. In the summer the weather gets seriously hot, we had days of 35*, but generally in the high twenties in the day. At night it would cool considerably due to the elevation, but never below 10* (I kept a thermometer in the hammock). In terms of rain, most days were sunny and clear, but with several blocks of 2-3 days of continuous drizzle, very Welsh..

due to being in the rain shadow of the Pyrenees. Several nights, especially after the periods of hot days, we had some seriously impressive thunderstorms and really heavy deluges, making it really muddy. (quick note on winter, went for a week in February and the temp was down to -18* some nights at 600m with some big snowfalls and freezing. :D )
Terrain - In general it is very wooded, probably up to about 1500m. I believe it was once predominantly pasture, but after the war the local economy collapsed and people mass migrated out of the area, letting the forest regrow. Most of the forest is therefore beech and smaller trees like birch and hazel, and only a few clearer areas of pasture remain, often accompanied by abandoned shepherds hut and barns. (not entirely legal but shelter possibly?

) I happen to live near Ludlow, and like Ian said, it is really rather like that area and moving out towards Knighton and Clun. Just a LOT bigger and steeper, very up and down and then up again. It makes for brilliant cycling and views though. Generally, there are both walking paths (the GR10 does pass through) and old drovers tracks. Most of which are rideable and in reasonable condition. I suspect it gets a lot more rocky if you go higher and south though. In the lower elevations hammocks would be perfect for pitching just about anywhere. It is very rural, and I vaguely recall reading somewhere that it is the least densely populated Department in France..
Water/Food - Where I stayed there were mountain springs just about everywhere. Water wouldn't be a problem, rather like Mid Wales. I drank from a spring without boiling or filtering for the 5 weeks and had no problems. There are quite a lot of small villages and hamlets. All of the slightly larger ones have a boulangerie and a bar. Not sure how comprehensive there stock is though? Some of the small towns have small supermarkets.
Okay, that's about all I can think of at the moment, if you have any questions I'll attempt to answer them as best I can. I hope this'll help. Apologies for waffling majorly..
(Oh and there are bears, lynx and wild boar prowling around.

Saw the wild boar on several occassions..)
Jack