As an owner of the Espson Stylus Photo 780, I too have only used the original Espson carts when I first bought it. After that, megatoners.com all the way. Thus my opinion on print quality is somewhat biased, as I cannot recal printing any photo quality pictures on the original carts (which come half empty) with any much sucess (I was still calibrating photoshop and the printer as well as my monitor).
However, what I can say is that I've been printing photo-quality (as well as graphics and text) for about a year and a half now, and have ordered from megatoners 5 times, for a total of 10 carts of color and 10 of black. I have had nothing but great experiances with the printing results. Even more appealing has been the service from megatoners.com, I don't think I've waited more than 4 days after ordering carts to recieve them, not to mention the unbelievable low prices (2 color and 2 black shipped to my door for $15... compared to $25 for 1 black that I have to pick up at the store).
My only quais with using generic inks: (and this is technically unsupported, so if someone could please clarify). I was reading about how Epson's inks have something called "dura-brite" or some marketing phrase like that, and how they are supposed to print pictures that keep their original color for decades to come if you use that ink and the epson paper. Now my question is, all these photos I am printing: if stored properly (no sunlight, low moisture, etc.) will they fade? If so, that worries me about the generic carts. Of course having no experiance with either ink cart brand, I can only speculate. But if someone could shed some light on this issure, it would be appreciated.