We wondered this for ages, and then in January decided to take the plunge and Milo arrived.
NAWT were fantastic and said if it didn't work out then they would take Milo back no questions. Milo arrived and straight away Jess knew there was a newcomer in his house, we had planned to introduce them slowly over the course of a few days, but Milo managed to escape out the bedroom that night and they met for the first time.
I think we have been VERY lucky, there has been chasing and playing even now, and I think they are still trying to sort out who is boss. The only hissing we've had was when Jess came home from his stay at the vets and was a bit sore and Milo wanted to play. Sometimes I wonder whether taking on Milo has upset Jess given what we've just been though with him, but deep down I don't think it has as they do seem to get on and we all cuddle up together, and Milo will spend hours cleaning Jess.
Its a gamble to take on another cat you will never know how its going to go, but with patients and being prepared for a little hard work it can be very rewarding and worthwile, my hubby was dead set against having another cat but his been won round now.