Don't go on shopping sprees at Harbor freight. A lot of tools are reviewed very well but some are not. I recommend checking out reviews for any power tools you might be interested in. It pays to be selective and return what doesn't work.
There are a few Pass/Fail Topics at various sites on the net that might be helpful:
Hand tools have lifetime warranties comparable to a lot of other companies. Hand tool quality is good enough for the weekend warrior and acceptable enough to supplement any better quality tools you might have.
Search around in the different coupon links in the above posts for different deals. The coupon pages have slight differences where you may find a coupon on an item you're interested in buying.
The 20% off coupon on top of the sale price might actually be better than the product specific coupons they send out. It pays to do the math.
Physical copies of the Harbor Freight magazine coupons can be found in the back of a lot of different Sports, Science, Auto, boating, wood working, and other men's magazines. A couple examples are popular science, car & driver, kiplinger's, motor trend, etc.
Some retail stores will pricematch the online prices via a printout. You might even be able to stack a 20% coupon on top as well (YMMV).
HF offers extended warranties on their products. I'm generally not a fan of extended warranties, but it might be worth it depending on the cost, product, and how much you plan to use the item.
ITC membership is probably not worth it for most at $29.99 per year. There aren't enough unique discounts and benefits that make it worthwhile.