We use Intertek. We are an engineering services and tech pubs company, but we do not do design. Our auditors have increasingly been a better fit, especially as we transitioned to ISO 9001:2015. Positives: they are large, offer multiple certifications, have auditors all over, and offer a lot of training.
Negatives: Communication and continuity. Each year the scheduling person is different, we rarely have the same auditor twice, I don't feel that their responses are timely but I may have high expectations.
My biggest disappointment has been their online portal. Their marketing department really plays it up, but it is not used up to its potential. I have put questions, scheduling requests, and personnel changes in the portal, but they don't generally get responses.
Overall, though, it's been a decent experience. Not outstanding, but sufficient.