Getting sued over negative internet product reviews

To be fair, don't confuse a letter with a lawsuit. There's a huge difference ... "legal threats" are meaningless. If I responded to every single one of them - which any lawyer will tell you to do (since that's how they make their money), I'd have wasted tens of thousands of dollars on legal fees over the years. Out of a dozen or so letters exactly like this, do you know how many actually went beyond sending that first letter? One. And even then no suit was ever filed, they just sent a second copy of the same letter.

For the most part, they are empty threats. Unless you know you are doing something wrong, there is absolutely no reason to respond to the first letter you receive. None. Knowing and understanding this will save you time and headache, because most people simply send the "demand letter" with the hopes of scaring you into doing what they ask out of fear of a lawsuit. Very few (less than 10%) will actually pursue it. And if they do, it's not before sending a second and third letter.

First letter - ignore it.
Second letter - Consult an attorney and ask if/how you should respond. Do not pay THEM to respond.
Third letter - retain the attorney and let him/her handle it from there.

Attorneys won't tell you this, they want to bill you to respond to the first letter. I made that amateur mistake once and paid an attorney $2,300 for them to "research case law" and then send a letter to someone informing them I would comply with their request. Complete waste of money.

People make the mistake of thinking an attorney is your friend, and they'll always give you helpful advice. That isn't true ... they're a business, just like any other business. Once you've retained them they are obligated by law to do their best job to represent you, but up until that happens they are "pitching their service" just like anyone else. So of course when you call an attorney, they're going to say "Yeah, this is delicate, let me handle it for you" - that's how they make their income. Anyone who's been doing this a while knows how it really works.

Your own individual stamina is a consideration too. Providing the review is properly worded, there is nothing a company can do to make you take it down. They can sue you, and you can spend a bunch of money defending yourself, but they won't actually win. And they know it - it's a bully tactic. They know that if they have more money than you, you'll cave and meet their demand, which is what they want. It's a mathematical formula, plain and simple ...

Product costs $X
Potential Audience of likely buyers who will see the Review = Y

If X(Y) > $Cost to sue you, they'll do it.
If not, they will just send letters.

That's it.

In the OP's particular situation, I still think the best way to handle it would be an offer to re-test if the company was willing to send a new sample, to ensure there were no issues with the previous sample due to improper storage, age, etc. You're extending an olive branch and giving them the opportunity to reveal whether or not they stand behind their brand - this will help tremendously if the situation does escalate further. Since we're talking about a product, they don't want a lawsuit. Lawsuits get press, and press gets more eyeballs on the negative review, and especially if the reviewer gave them the opportunity to re-test using a sample acquired directly from the company for the express purpose of performing the test.