I'm with the 121.5 Pan. First time I called it I was lost through a mistake in reading my flight plan compounded by strong, unexpected winds. The guy was brilliant, spoke clearly, led me by the hand until I was happy with my situation, offered a radar information service, vectors to where I wanted to go, confirmed my QNH & height and gave constant updates of my precise position to make sure I knew where I was and where I was going. I had his undivided attention for a good 5 minutes.
My only regret was that I didn't call them sooner, as soon as I had any doubt about my location. Instead I stumbled around for a good few minutes, which was daft.
Having called once, I'd have no hesitation calling if weather started looking tricky and I had safety worries. I have no doubt that if you call them you won't regret it, but best called early rather than late. Great if you're fixed wing - they would be less useful in rotary as once you're in cloud as a non-IR PPL(H), the instability and lack of experience means your life expectancy is probably a small number of minutes (1?). I'm frankly too scared to go anywhere near them.
BW