Just give your opinion as you would to a coworker on your best day. That's what any reasonable interviewer is looking for unless otherwise stated. Similarly, you can tell a lot by how your comments are received.
Of course you should be polite in an interview - but for a desirable employee it's not only the company judging the employee, it's the employee judging the company.
And if the employee sees a sample of the company's work and it's awful, they're liable to judge they're unlikely to learn much from working there.
