In terms of employment, it's highly dependent on field. Some fields, like law, really care where you did your degree and generally getting a pupilage etc is much more difficult if you didn't go to a top 20 uni (not necessarily Russell Group).
In some cases, if you want to progress in accademia/research then Russell Group will help.
But equally in some fields it doesn't matter as much, and some fields employers are more interested in highly rated unis for that subject, rather than just "Russell Group". For example, for art, Goldsmiths is generally considered the best place to go, Loughborough for sport etc.
Some employers like highly specialised degrees too - the most successful person I know (financially) did a specialized degree at Nottingham Trent and now works in the energy sector as a consultant earning really good money. I know a lot of people who went to RG unis who earn a lot less.
In terms of teaching quality, it certainly used to be the case that at Russell Group unis, you were more likely to be taught by pHD students etc, whereas at more teaching focused unis, you'd be more likely to be taught by more senior lecturers, at least at undergrad. Most of those lectures will still do research and may still be leading in their specific field.
Definitely in terms of earning potential it depends a lot more on the degree you do, and the result you come out with. I think it used to be the case that doing eg History at a well respected uni would open a lot of doors in eg business, but I think that is getting less and less to be the case and many employers would now prefer the person with the degree that's more specifically tailored to their field or offered a year in industry etc.
At this stage I do think the RG is mainly a marketing group, but because it's successful it does still influence some employers. But if it were dissolved tomorrow then I think employers would still respect a lot of unis on the list.
But do people really rate, say Southampton and Sheffield over Lancaster and Bath? I'm not convinced.