I was in the RCMP for about a year and it is in my experience that clans can be great if you block out the childish focus on unimportant things like ranks. I have made a lot of friends in that community and a few whom I almost consider family now. Along with Gunner 9213, I would agree that having real-life backup instead of AI is the coolest thing ever and you can get into some great roleplay when you have twenty some members in a game. It's definitely something you need to try but it's not for everybody. Do make sure that you research the priorities (i.e. member activity, realism, professionalism, relaxation) of each community you are considering to see if they coincide with what you want and give it a solid effort before deciding to stay or leave (I suggest 3 months).
That being said I am no longer in the RCMP and my fellow ex-RCMP friends and I are wanting to create a small group to play. So if the clan doesn't work out for you feel free to pm me.
One last thing, unless you are willing to dedicate 40+ hours a week to developing and managing a community in addition to recruiting about 20 people to start it with you, the chances will be significantly low you will reach any kind of success especially at this point in GTA IV's life.