3rd party transformers..

[Ode to a Grasshopper]

It's very much on a case by case basis for me.
I paid a horrible amount for City Commander + Classics Ultra Magnus, but it's a beautiful piece and the enjoyment I get from it even now has made it worthwhile IMO - much like MP10. The Headrobots RotF Bludgeon upgrade kit was kind of pricey, but again worth it. Ditto for the Beelzeboss Goldbug set. On the other hand, my PE not-Reflector (the black one, 'cos I couldn't find a purple set at the time), while also a lovely piece, hasn't quite delivered the bang for the buck I would have liked - though it was from a Japanese specialty shop rather than a more economical option, so I might feel differently if it weren't for the Hero Gangu markup.
So far I've avoided combiners, but I'm a sucker for Predaking so I'm probably gonna cave for MMCs Feral Rex crew, and I suspect it'll work out much the same as City Commander - expensive, but ultimately worth it IMO.