1. Is there a better/cheaper alternative?
Depends on your needs

There are cheaper ones, but less reliable or less features
2. What are the problems/annoyances you experience with Visonic?
Communication protocol is closed. PowerMaster system don't work properly anymore when connected serial (e.g. PIRs are unuseable).
3. What's great about it?
Reliable
4. Why have you switched? (and to what and why?)
Not yet, but i won't buy another Visonic
5. Are you planning to switch? (to what and why?)
Not yet, but thinking about a custom build e.g. with Fibaro
6. If you had to pick a new alarm solution what would you choose? (Visonic, or ?)
Custom build

DomotiGa / Z-Wave / RFXCom / Visonic PowerMaster / Home Assistant / Zigbee2MQTT / DSMR Reader / Toon1 (rooted)