Hi,
I have used Hermes for quite some time. They are much cheaper than Royal Mail for larger items, but I've recently found out they use "Joe public" to deliver to your door in some instances.
I've only had 1 complaint regarding delivery, but then my items aren't necessarily time sensitive.
I do know of some businesses who no longer use them as they considered them unreliable.
I think if you an individual selling items "via popular internet auction sites" then they are great, as the cheaper cost will help the sale, but as far as business goes perhaps not.
Look at a website Parcel2Go as from here you get a wide choice of different couriers to choose from, and the user interface is simple.
I should point out I'm in no way affiliated with this website.
I hope this helps.