One question that I see frequently is if PLR products are worth buying at all and if so, which ones?

The short answer to that question is that SOME private label rights products are worth the money, but not all. 

For example, I'm a big fan of purchasing private label ebooks and using them as is as a free "bribe" to get people to sign up for my newsletter.  Frequently these products are good enough that if you spent a day adding a little more info to the product to make it unique, you could charge for it. 

The sales pitches that you see for these products that you can instantly have a lot of products to sell without doing any work does tend to be a little misleading since you're going to frequently have a lot of competition selling an identical product.   My advice is to take the time to make your product unique or hire someone who will do it.  It takes a little longer than using the product as is, but it is a heck of a lot faster and cheaper than starting from scratch.