I usually prefer to make my own combinations, and I have only bought one jelly roll. It was an impulse because of a good price at Tuesday Morning. It was a Kathy Davis design for Free Spirit - excellent quality. I was pleasantly surprised with the fabrics I didn't see in advance, but that was mostly chance. Although the line had been discontinued before I finally decided what to do with it, I was able to find several additional pieces from the same line on Etsy for borders and backing, so that was lucky, too. I don't always enjoy cutting strips, and can certainly see an advantage, but at full price you usually pay quite a bit extra because of the cutting and packaging process. One of the big advantages is that many stores will not cut less than a quarter yard of fabric, but with a JR you can buy multiple narrow strips without having to pay for more fabric of each design than you want. I wish Tuesday Morning would frequently offer precuts, but usually I don't find any there.
There are 14.4 2 1/5" strips in a yard, so you could do the math when you see a JR. Divide the number of strips by 14.4 and that will tell you how many yards of fabric you're getting. A 40 strip roll is about 2.8 yds of fabric. You could divide the price by 2.8 and find how much it is per yard. It is worthwhile to carry a calculator or a smart phone with a calculator app, unless you're a math wiz or just don't care how much you spend.
Any time you're thinking of buying a jelly roll from someone who doesn't provide a picture of all the fabrics, use Google image search to see what the line consists of. Double check to make sure it's not just showing you an irrelevant picture, but usually someone will have what you're looking for somewhere on the Internet. It's saved me from buying some I might not have liked, and also helped me to see what is available to go with the fabrics I do like.