Been using Opus 4.6 daily for the past week or so building an MCP server. The agentic task sustain is real — it holds context across much longer multi-step implementations than 4.5 did. The adaptive thinking is a genuine quality-of-life improvement for mixed workloads where some calls need deep reasoning and others are straightforward. One trade-off I've noticed: it can overthink simple tasks at the default high effort setting. Dialing effort to medium for routine work and keeping high for complex problems has been the right pattern for me.