All depends on what you are looking for, my thoughts are the likes of the ESS etc are good but for a more industrial rated machine a better option is one of the CSMIO products.
My first choice is the CSMIO/IP-A but that requires that your servo drives can accept analogue command (+/-10v)
If they are only Step/Dir then the CSMIO/IP-S does that, just the IP-A has a few more benefits.
If the I/O requirements are not so high then there is the lower end CSMIO/IP-M which again is Step/Dir.
Hood