You've already worked with this company's product, and you know it and like it? Yes, YES, you should mention this! You have a fantastic opportunity here, one that other applicants might not have. Don't waste it!
Spend a minute imagining that you are the interviewer rather than the interviewee. All things being equal, who would you rather hire? Someone who will have to spend days or weeks learning the product--and might even come to realize that they dislike the way you do things? Or someone who has already worked with the product enough that they can hit the ground running on day one, excited and knowledgeable?
All things being equal, when choosing between two similarly qualified candidates, there's not a company in the world that wouldn't prefer to hire this second choice! It'll save them time, money, and hassle.
And remember, your interviewer doesn't just want to know that you'll be good for the job. She'll also want to make sure that you will stay on the job! Turnover is a beeyotch, and it's extremely common nowadays. If someone isn't a good fit, they won't last long.
Mentioning that you know and love the product, then, will be a huge feather in your cap. Because it doesn't just say why you would be a great fit for her company: it also speaks to why it might be a particularly great fit for you. If you already know and love the product, that means you won't be as likely to walk off the job the moment some other offer for $0.35 more an hour comes your way.
It also hints at a more pleasant and productive work environment, for you and your whole team. You won't be some Negative Nancy, constantly butting heads and demanding they change course. You'll be a Helpful Harriett, someone who clearly likes the voyage they're already on! And that doesn't mean you're just "along for the ride," either. But it does mean that when seas get choppy, you'll be more likely to help steady the rudder than to abandon ship.
This kind of "fit" is so important to companies that they are far more likely to hire someone who is a "good fit" over someone with a theoretically great resume. And this is true even if your position isn't at the core of what they do--e.g. if you are applying to this company as an accountant or a salesman rather than someone who will literally be building the circuits.
Now, I agree with other comments on here that you shouldn't just say nice things about their circuits as a weird non sequitur. Don't show up at the interview and say, "Hi! My name is Chris and by the way if you change your circuits I'll KILL MYSELF!" And don't assume that giving a glowing review of their product will do all the heavy lifting during the interview process.
But during that interview, several opportunities to bring up your knowledge and appreciation for their product will arise naturally, as part of the discussion. Take one!
And when you do, make sure to talk about their product in a way that reveals your knowledge of it, not just your appreciation of it. This will separate you from the fakers and the haters.
It's not enough to clinch the job, but it will be enough to help you stick out from the pack. At the very least, it will make you sound eager to impress, and that's rarely a bad thing.