I’ve used them three or four times with great results. Seems to good to be true but it is true. Here the USPS rates through pirate ship are no bargain, however the UPS rates are fantastic. I have found the key is to keep the box length below 49” if possible. Seems once you hit that magic number the rate jumps about 10-12 dollars. Seems more crucial than weight. Just make sure you measure and weigh the package somewhat accurate. I’ve heard they will charge the recipient if more money is due on the delivery because of errors.
Just to offer an alternate data point, the USPS rates through PirateShip are consistently a couple dollars cheaper than using my local PO.
This is probably true I have just found the UPS rates for the exact same package are much cheaper than the USPS rates for the same package on pirate.
Between eBay and the various forums I've been on for the last couple of decades, I've shipped many hundreds of items so am pretty familiar with the different shipping companies and their various practices.
Each of these carriers have their retail rates that all of us average joes pay and they also have a number of business/commercial accounts that offer varying levels of discounts depending on how much you ship. What PirateShip does when you ship through them is give you the discounted business shipping rates without you having to have a business account yourself.
The rest of the stuff around dimensions, weight, etc. is passed through and enforced by the carriers themselves. For example, I used to ship my individual golf clubs through USPS Priority Mail because it was the best balance between cost (~$15) and delivery time (2-3 days). Recently they changed their pricing to dramatically increase the cost of shipping longer items over 24" (or maybe it's 36", I can't remember) to be almost double what it was before so now it doesn't make sense to use USPS Priority Mail for golf clubs. At the same time, FedEX thru eBay dramatically reduced their 2-day shipping to many states so now it makes more sense to use FedEX for those.
As for dimensions and weight: most carriers will have a surcharge for larger packages. Once you exceed certain dimensions, they no longer charge you the actual weight but, instead, charge you by "dimensional weight." For example, a 28"x20"x12" package might actually weigh 7lbs but will be billed at, say, 16lbs due to the oversized package. Also, if you're purchasing the label yourself online (whether through PirateShip or the carrier's website), whatever dimensions & weight you enter when you purchase the label, they can sometimes spot check and verify the size/weight of the package and if you entered dimensions that are smaller or a weight that is lighter than the actual package, they will charge you the difference. But this is done and enforced the carriers themselves and NOT by PirateShip - so don't blame PirateShip for these after-the-fact charges, it's not their fault - it's yours for not putting in the correct data to begin with.
All in all, PirateShip has been phenomenal to use. It's not only cheaper most of the time, the process & workflow is much easier compared to the UPS/FedEX/USPS website (there are like 7,000 clicks to generate a shipping lable on the USPS website).