Just finished my first century on my cx bike. It is extremely comfortable and has all the performance you need for fast group rides unless you get into serious racing. I ride 28c slicks for these events and 35c cx tires for trails. I also ride about 50miles with my cycling team averaging 20-22mph and do fine. Now if I were riding with the competitive racing group I would want something different.
My bike gets group rides, organized centuries, trails, and pulling my kids around. I'm very pleased with it. I'll eventually get a dedicated road bike so I don't have to keep switching up my setup but I don't feel like I'm missing out on anything by having a cx bike and I've gained a whole lot of versatility.