It is simple and just works. No tweaking or jacking with it.
Does not sync contacts without email addresses
It is pretty plug and play. I have deep integrations set up in mere minutes with this tool. Imagine two columns of contact fields, and a drop-down for each field. Once you've worked out your mapping, it's smooth sailing, and updates are nearly instant.
I am a heavy user of SMS, though, and really need it to sync these contacts into an SMS automation tool for nuturing. A Twilio integration would be awesome for this.