We get asked this question all the time and our answer is always the same its your choice that's the whole point of the products being available to give the marketplace a choice. That said sometimes you have no choice some manufacturers insist on genuine accessories to keep warranties intact as the accessories have been developed in tandem with their products to ensure correct fit, performance and longevity.
Now don't worry there's more than that neutral statement of course we have an opinion ! , based on our 40 years of experience you really can't beat the genuine article, we are very fortunate that we are a large independent distributor that isn't tied into any one manufacturer so we can supply genuine parts and accessories for all the major manufacturers so this is what we try and do. That said for many reasons (cost/availability/usage) we do sell a lot of equivalent spare parts and accessories and on the whole the quality is very good for the price and in lower usage situations or a particular nasty job that means you want to throw the tools away after they are the cost effective choice.
One last addition is vacuum bags yes pattern spares are cheap but they really can't compare with the genuine products Numatic (henry) , SEBO and Nilfisk have done extensive testing with genuine vs equivalent bags and the filtration level of genuine bags is far higher so not only are there less particles able to escape back into the atmosphere they also ensure that the filters and motors of the machine are kept dust free and again will last much longer. With the average cost of a motor around £80 is saving £2 or 20p per bag on a pack of 10 bags really worth it ?
As we said from the very beginning it is of course your choice