I usually go during the half term week, but it is usually the 2nd week that is the quietest. It appears that most counties are off school the second week this year. So either week could be busy and looks like I've picked the worse week out of the two for my visit
Some schools have just over a week off and could overlaps into both weeks.
Yes worth going, but plan well and don't follow the crowds. Do the most popular rides first (first thing, get in early as queues can build up within minutes) or last thing.