I have had all IW (506, 510). They are OK, but I feel the build quality is diminishing and the price is increasing. For that reason, I looked into other brands and the one that came out on top for me was Cheval Liberte. I ended up opting for a 3.5t because I was sick of the car costing a fortune but I'd have gone for a CL if I was to continue with a trailer.
I've done a lot of research into this recently and decided to keep my 16 yr old IW510 for the time being and keep an eye out for a second hand Bockmann (and a specific model at that). They're like hen's teeth so it's just as well I'm in no hurry. Everything else is either pretty rubbish or costs an arm and a leg or both.