I've used Neoflash carts, Everdrives, and a NES Powerpak, and right now unless you can get the others cheap I probably wouldn't buy anything but Everdrives at this point. The Chinese stuff has weird issues, and the Powerpaks are nice but the basic Everdrives are quite a bit cheaper. The only downside is you need to dremel a cart shell or overpay for SAG to do it for you.
By coincidence, I have a Genesis Everdrive on the way, it should arrive today!