The cashback worked painlessly for us, yes you have to jump through the hoops correctly but we got paid without issue and ahead of the timeframe HP gave themselves to do it in.
With the Instant Ink it's worth noting that you can have a full month's worth of pages in the bank as it were.
We're on the 100 pages per month plan which costs £3.49 per month, but if we print 50 pages in Month 1, next month we have 150 pages, but you can never have more than 200 pages in a single month on that plan without paying extra. (Which is £1 per 20 pages, although we've never gone over our limit.)
Also the cartridges HP send are mega jumbo bastards and last for ages. When they are getting low, the printer phones home and HP just send more in the post without any intervention from us.
Overall we've cut our ink costs by about 40-50%, and are on a far more convenient system than we were in the past. (And also because you pay for page numbers, there's no stress about printing a load of pages with 100% ink coverage or suchlike.)
It's a great system and it works well.