If it is FTTH then the IP transport should be reliable. The complaints fall into two main categories:
1) Problems with firmware on the ZTE set top boxes. These boxes were originally used on FTTN installs but are also turning up on FTTH installs. The software has been improving but slooooowly.... One can only hope that CB have threatened ZTE with appropriate action if they don't improve matters.
2) Problems with VDSL. Only FTTN installs use VDSL. CB aren't the only carrier suffering with VDSL issues. One problem is that as more customers are added a particular pair bundle may suddenly have new pairs running VDSL and causing interference to existing VDSL pairs. A customer whose service had been OK suddenly becomes unhappy because their service suddenly gets worse. Vectoring provides some mitigation by monitoring multiple circuits and determining the appropriate signal levels to reduce crosstalk while maintaining throughput performance but I'm not sure if the Calix gear CB is installing supports vectoring.
If you are being offered FTTH then you will probably love the Internet and be prepared to put up with the ZTE boxes for a while.
If it turns out to be FTTN you will *probably* be OK because 30 Mbps down is only offered relatively close to the fiber node so your chances of crosstalk problems are lower than some. The fact that your DSL connection has been reliable also suggests availability of a decent copper pair for a VDSL circuit.