As a long term HP user, I agree with seascue. It is just a shame that the capacity of the later HP cartridges is much less than their earlier ones.
My old HP 820Cxi uses a No 45 (Black) cartridge with 42ml of ink, and a No 41 (Tri-Colour) cartridge with 40.5ml. Our HP psc950 uses a No 15* (Black) with only 25ml, and a No 78 (Tri-Colour) with 19ml. The No 17 (Tri-Colour) cartridge is smaller again, with only 15ml.
(*Beware also that there are two No 15s available. The C6615NE may look a bargain on price, but it only holds 14ml)