I haven't used the CIS system, but did use good quality aftermarket ones on the Epson printer I used to have - bought from the PC shop I bought my printer from - found them to be a mixed bag really - cheaper yes, quality nearly as good, but dried up much quicker in the nozzle than the genuine Epson ones - in the end, the shop replaced the nozzle etc for me as I was having problems and they had recommended these to me and I stuck with genuine ones after that and had no problems...
Could have been a duff batch, but ...