I got the same offer in the mail and decided to take it; went over yesterday to sign up. I don't see any downside to it. Even though my membership for the year is free, I was able to add a second person to my account for free, so I added my sister, and she appreciated it. You would add your spouse, I assume, but apparently it's not limited to household members.
I do still like Costco but sometimes don't want to drive all the way there (I live roughly midway between the Mettawa and Lake Zurich Costcos). The Vernon Hills Sam's Club is half as far from my house as either Costco. There's also a Sam's Club in Gurnee I can see visiting. I just looked around yesterday after I signed up, but I'm going back in a day or two to buy gas, which, like at Costco, is about 20 cents cheaper than at gas stations and therefore about the same as Costco but, again, a shorter drive.
On the weekday afternoon that I was at Sam's Club, the differences between Costco in terms of in-store crowding and checkout lines and outdoor parking availability and lines for gas were noticeable and welcome.
(Would just add as a personal reflection, I hadn't previously been in a Sam's Club for more than 25 years, when I lived in Champaign. But now there are both a Sam's Club and a Costco in Champaign --- and a Portillo's too --- all of which I will be grateful for if I end up relocating back to C-U someday.)
"Your swimming suit matches your eyes, you hold your nose before diving, loving you has made me bananas!"