I have had to join Mumsnet to try to balance this.
My children went to Connaught House until recently, and were very happy and did very well. They were not boys, but they are still friends with most of the boys that were in their class, who all have happy memories of it. I don't think the boys were bored - to a person they all went on to have places in top London schools and I don't know of any tutoring in our years. They all have happy memories of being there : we know as we hear them reminisce.
In many ways the boys did better than the girls in their movement to next schools.
The thing that is difficult is that it is a small school. If you have a good class (our children all did) who do lots together, all is good. If you have a more difficult class where the parents are in some way disatisfied, because there aren't enough people, it is very easy for disatisfied parents to sway others.
You have to remember that boys are only there until they are 7. They have a happy time, learn a lot, and do a lot. It is a safe environment for a young child. By the time they are 7 or 8 they are ready to move on - but then that is the point of the school : to prepare you for prep school.
There are lots of things that are good : the academic standard is high, the art is good; the behaviour is good. They are in a small but nice building very close to the park so they get fresh air and to run around.
If you choose a small school you have to appreciate what is good about it (personal attention, central location, local families to socialise with), and accept the limitations of a small school. Also try not to listen too much to playground gossip - or at least try and stand back from it and make your own decisions.
Also, it is not that selective: so it is good at getting the bright children into teh top academic schools - but also, if that is not the right place for your child, finding somewhere that will work for you.