First, let me say that I'll only be speaking of the HO scale products offered by Bachmann. It's not easy to make a blanket generalization about their entire catalog, but classifying the models by the product line can make it easier to rate them. They have a standard Bachmann branded line, a Silver Series, a Bachmann Plus line and a Spectrum Series line. Their Bachmann line is pretty much toy train set fare and is among the best in that category. So, considering the competition in the toy train set arena, I'd give them a 6 out of 10.
Their Silver line and their Bachmann Plus line are an attempt to compete with the manufacturers that dominate the level above toy train set stuff, but they are pretty much at the bottom of the barrel there. The diesels in the Bachmann Plus line are about the poorest can motor and flywheel equipped models available, lacking in both performance, detail, paint and fidelity to the prototype. Still, they are a step up from the train set line and not that bad, but virtually anything else you buy in this category will be better. 3 out of 10 against comparable products.
The Spectrum line, which is intended to compete with the Proto 2000 line by Walthers (formerly Life-Like, which is now owned by Walthers), the Master Series line from Atlas and the Genesis line from Athearn, is very hit or miss. They butchered nearly every diesel they modeled, in terms of detail proportion and fidelity to the prototype. Separately-applied items like handrails, cut levers and horns are oversized, flimsy or poorly color matched. But the worst thing of all is how poorly they all seem to operate. So, for the most part, the Spectrum diesels get a 1 out of 10.
The Spectrum steamers however, are another story. They are everything their diesels are not and range from 8 out of 10 to off the charts good.