Generally I'm pleased with my purchases and as I own a considerable number of locos over 1000 I can tell the difference between a good one and a not so good one.
I recently purchased 2 blue and 1 green 22 which have to be weathered to improve that awful colour they came in and I have 2 Westerns. One runs like a dream but the second shakes like a jelly on a plate and stalls over every set of points. This one I've return to Kernow costing over £10 in postage, I doubt I'll see that expense refunded.
I feel that the cost of locos today does make this an expensive hobby, so if there is a problem with an item them it should be returned, first to illustrate the issue to the manufacturer and secondly to avoid the expense of repairing locos to the customer.
Finally I agree with many on here in that quality control should be carried out at source and not by the customer but also some do escape the net, which is when a manufacturer must excel in customer service and not become critical as I'm sure most don't want or in reality deserve negative reviews as most models are fine.