I run an IP5200 and until recently would never, ever had considered compatibles but I'm now on my second set of 'Think' cartridges and I'm completely sold having seen no difference between these and Canon - apart from the price!!!
I have an ip4600 & (apart from the first time) have always used compatibles purchased from eBay with no problems. Each time the cartridges have arrived individually boxed + sealed & are less than half the price of Choice/Think cartridges. G
Theres a range of compatibles out there to suit all requirements and budgets, the rule I tend to stick to is know your expectations. If you just need basic printing (perhaps just every day stuff) unbranded 'economy' type cartridges will do the job, however if you need a little more from your printing or print a lot of photos, go for a branded cartridge such as the JetTec as mentioned above in the other posts! Selectafont.co.uk offer a wide range of branded/non branded compatibles and I've just recommended them to my sister who's doing a photography course and she's delighted with the print quality of the JetTec cartridges she received.