My wife and I both come from truck camper families so we are biased.
When my son was born in 1992 I bought a truck and camper (92 Isuzu and a 75 Okanagan import camper). In 94 my daughter was born and we continued using the OK camper until my inlaws sold us the 96 slumber queen import camper in 2002 (we did a lot of camping with them). In 98 we got a border collie cross who also joined us on many of our camping adventures. Yes the old OK camper was crowed, but we had lots of great adventures in it. In 2006 we upgrade to our current rig as we wanted to go across the country and my 6'2" son no longer was comfortable in the old truck. With the new camper we cut a piece of foam so my son could sleep on the floor - too tall to sleep in the dinette bed. We spent 7 weeks in the camper crossing the country.
Before we got the larger camper we had done a couple week long back country trips (2 - 2 man tents). The amount of space you need is relative to what you are use to. For our family of 4 and dog, the camper provided just enough space. On rainy days we still went out (awning or tarps help), if it was raining really hard the kids loved the opportunity to play in the overhead bunk.
The advantage I see in having the truck camper is that I can park it almost anywhere, turn it around if you take a wrong turn and it costs substantially less to take a ferry.
As my kids are now grown, out camper seems huge when it is just my wife and I camping. Last summer my 19 year old daughter and I went on a camping trip to Idaho as she has gotten the truck camper bug. My 21 year old son still will meet us for a weekend of camping and still sleeps on the floor.
The camper for us has been mostly for sleeping in. When we are camping we are out exploring.