Yes, a few times, and enjoyed them all (last year was at Lemonos), and is nothing like a holiday camp. Taking the positive aspects first -
It is very well organised, both for the activities and the general running of the hotel. The activities staff are all British (apart from the waterski people who are usually locals due to licensing issues), and are some of the most helpful, friendly and pleasant people that you will ever deal with.
The first time I went on a Neilson holiday I had never been near a dingy or windsurfer and was more than slightly concerned that I was outside the age / fitness profile of their average customer. I need not have worried, the activities are organised so that people are split into new / medium / expert groups, and the staff are very skilled in getting people to learn new skills, but without forgetting that people are on holiday.
As for getting out and seeing the locality, if you don't turn up for a lesson or an event, because you have decided to do something else, nobody is going to even mention it. The meal plan (all lunches and 4 evening meals included) also encourages you to get out for several evenings, but also allows you to relax on other nights and not have to find yet another restaurant.
Also if you take some of the bike tours you will get to see quite a lot of the surrounding area.
Now for the downsides, apart from the horrid charter flight, the only downside it that with they type of person Neilson attracts, it can feel a bit like 'Waitrose on Sea' at times. On more than one occasion I have seen mummy & daddy puzzled by what to feed the little one at breakfast, because the nanny normally does that.
But overall, if you throw yourself into it, if you totted up at the end of the week what it would have cost you for all those tennis, dingy, windsurfing lessons, hiring tennis courts, dingys, windurfing gear, bikes, etc, it is a steal.