Wolfdale, there are other iPhone forums. I can't say how popular any of them are compared to each other, but the iMore one is particularly good. However, I find this to be the best and most informative iPhone forum. (Really!) If there is something iPhone that you want to discuss, I'd be very surprised if you can't find a relevant thread about it here... While, if you do have a problem with anything regarding your iPhone, you will find plenty of helpful people here too.
All forums go through quiet periods. I'd say in the run up to a new iPhone launch or major iOS update you will notice a spike in interest and plenty of activity here. So stay tuned...