We have just such a dog (click on my name and see a picture in my photo section!).
Whilst he is lovely and has many admirable qualities... I would caution you with the advice that I personally will not be getting another one once he is no more!!
I exercise him A LOT... but after half an hour's R & R it's as if he'd never been out. These dogs have been bred for stamina - working on a shoot all day - and boy do they have buckets of it!
Added to this we have had to put 6 foot fencing around our garden becos he runs off at any opportunity and roams... this is a common trait of the breed and I know several other owners who have had to install similar concentration camp barriers! In fact the kennels where he goes whilst we are on holiday have special exercise pens for the pointers as they were escaping and "climbing" the fences in the other paddocks! They do actually use their front feet like a cat would use it's claws so they have tremendous upper body strength and can scale ridiculous heights if they are so minded!
They can also be incredibly highly strung verging on the neurotic... infact a lot of people I have spoken to who work gundogs won't use gsp's because of this.
I guess I would want to know an AWFUL lot about the dog before i got a rescue... I would imagine that the reasons it is in there will be the things that could in turn drive you mad... how old are your kids, how much time do you have etc. We had our dog from puppyhood... seemingly with out bad habits etc and that was hard enough.
On the positive side they are usually very good natured, good with other dogs (can be dodgy with cats, although ours lived with our cat until it was run over) good with kids on the whole, always up for a walk etc. So there are lots of great things about them!
I realise this is a negative response but the re-homing rate on these dogs is really high due to the reasons I've listed... I'd wager it's why he's in there now. I'd probably be just as tempted as you to rescue him!