Some are, some aren't, like anything it can be a mixed bag.
Spent a fortune going to The Yeatman, and it was absolutely amazing and the best food and dining experience I'll probably ever have. Even though it was hugely expensive, I actually think it was worth it for the quality of the food, the variety and the experience. I had the tasting menu and I swear it was about 13-14 courses and I had the paired wine flight with that too. Just amazing. The attention to detail and flavours in every course were off the charts... Not to mention the Port, wow.
I've been to quite a few in the UK as well and some were excellent (like Benares) and others I was really disappointed in (like Murano).
If you want to get an idea of what it's like, the best way is to grab some lunch at one that takes your fancy. Often there are fixed price lunch menus and they can be very reasonable indeed. I'm hoping to go to Marcus next and do just that.
Now, before people think I'm a fine dining snob about this, I'm not, I just genuinely think food can be amazing, but I've had many other amazing food experiences, like eating market food in Cambodia in the middle of the night which probably cost me pennies but was still fantastic in its own way. It's often about the whole experience, as well as who you are sharing it with. There is absolutely a place for fine dining but it isn't the only way to enjoy food, sometimes the most innocuous basement tapas bar or food stall can leave as much a positive impression! However, if you haven't tried a Michelin star restaurant, I'd say it's always worth a go and they are more accessible than you think.