I think it's be best money you can spend on your hobby. All the 'what tyres for xxx' stuff when really the cost of a set of tyres someone can show you how to use them properly 🙂 A few years ago I went on one, quite reluctantly because I was as good a rider as anyone I rode with (I thought) and was normally quicker than most people etc etc etc. It made a massive difference to my riding, ironing out some bad habits and also giving me a list of stuff to work on. It was like my riding had got quite far but had taken a few duff turnings and ended up at a cul-de-sac. I had to reverse back up the road a bit, take some different turns and all of a sudden there was a lot more road infront of me for me to progress down.
Don't expect it to make you able to wheelie though! I think skills courses give you a set of things to work on but it's only practice that turns those into actual improved riding.
There's loads of skills courses about and they many seem to get a good write up. I've got some skills weeks with Jedi (uk bike skills) next year so I don't feel I can offer an unbiased recommendation other than to say that I approached Jedi because I've ridden with him before and felt he would do a good job!