An important point is that not all PhDs are equal. It varies a lot depending on the advisor, the subject, and the lab where it takes place. Some topics are quite "niche" and will not be useful in the industry. Some advisors don't really supervise their students (can happen in the best schools, with reputable professors). Check to see if you'll have the freedom to visit other labs/companies during your PhD. It's good to see a variety of places to get ideas from people, initiate collaborations, build a network. It's usually possible, but requires initiative from your part. And you need to make sure you will not get stuck in a never ending, boring PhD.

> I'm just not that bright and that a PhD would be a huge intellectual challenge for me

Don't think you need to be bright to complete a PhD. If you're able to get accepted in a program with a decent advisor, you should have what it takes to complete your PhD. It's also a matter of personality, perseverance.

I'm pretty happy that I completed a PhD. For one thing, it's great to have more than one professional life. I did a variety of things. Worked in academia, in several labs, several countries, and now in company. I had friends who went straight working for a company after their Msc. They earned more money, but I have the feeling I had a more diverse career, and that I learned more things (even though some of them are pretty useless). On the other hand, even in the industry, there are many opportunities to do interesting things. It's up to you not to get stuck in a boring job.

There's no definite answer.