I have always had one, and they are SO worthwhile. Unfortunately I have had to use it multiple times - kids!
You need to update every three years, and should do a CPR refresher every 12 months to stay current. I find that the more times I do it, the more confident I become, which then makes me calmer in the situation, always a plus.
So, in 10 years, I've gone through a variety of providers. I can't go past St Johns for being the best. Red Cross were also good. The others all mediocre, but they're cheap. Good for refreshers.