I had a HP printer recently, a very simple no-frills printer, excellent quality, fabulous prints. When the price of a replacement pack of one colour and one B&W cartridge rose to almost £50 (ten pounds less than the priner cost me) I had to think again. I could not find anything cheaper anywhere, here, on ebay, in the shops, anywhere. The compatible ones are rubbish, either low quality or dont work at all, or patchy performance. I bought this Kodak ESP 3250 because I needed a scanner too and the cartridge prices are much less. You do get less ink than the HP cartridges, probably about half as much, but as the HP cartridges are more than double, you still aren't being ripped off so much by Kodak as you are by HP.
Update: the older your printer gets, and by 'old' I mean more than 6 months, the thirstier it becomes! Suddenly those cartridges aren't so economical anymore!