It obviously depends on how often you shop at harbor freight but it definitely can be worth it. A lot of people here saying "it's not as good as the 20% off coupon" but the pervasive 20% off coupon doesn't exist anymore. Also, if you're casually paying attention to HF, you can you're not going to appreciate the frequency and extent of the ITC member deals.
I recommend getting the app. It notifies you of weekly deals including all deals and ITC member deals. It basically doubles the number of items that come on sale. Also, when the do have a good coupon, there is usually a larger discount for ITC member. For example, right now through tomorrow, there is a 10% off any single purchase (no exceptions) coupon that is 15% for ITC members. On a toolbox or something large, that's a real difference, but again, if you only buy one item per year, it doesn't make sense.
Also, I shop there often enough it's nice not to have to actually bring the coupon in. It's all just ties to my itc member number/phone number so I automatically get whatever discounts are available when I check out.
HF has done the math and figured out that they're going to make money on the program because most people won't save as much as they spend on the membership. You just have to figure out whether your spending habits put you in the category with those people or the few of us that spend enough to actually benefit.